Before you log on to ebay, start prowling garage sales or haunting flea markets, ask yourself several questions: What is my hobby? Is it woodworking or restoring rusty, broken relics from days gone by? Am I experienced enough to differentiate a good plane from junk? Can I afford to spend some money getting an education? Do I have the metalworking talents and tools required to resurrect these artifacts? Will the price still be a bargain once I replace the blade or missing and abused parts? If you can respond honestly "yes" to ALL of these questions, then you may derive satisfaction in bringing these good old guys back to life, and when you use them it will be "shaking hands with the past". If the answer to even one of these questions is "no", then save yourself frustration, and probably money, too, and purchase a new high quality plane from a reputable dealer.
Some sellers will return your purchase price, but you may be out 2X the shipping and handling charges. For a common plane this may be more than it's worth, so it's best that you make sure you want the plane and are sure you know what your are buying before you bid. Also, don't forget to check the shipping and handling charges the seller may be charging before you bid. I've seen sellers attempting to charge $60 or more for shipping a common bench plane. Also, make sure you are buying a plane that is already in the U.S. Shipping from England or Australia and your losses in exchange rates may be more than the purchase price.
Patience, patience, patience
This is the real key for ebay shopping. You have to know what it's worth to you and be prepared to fold if the price goes too high. I can guarantee a similar or identical plane will show up at a reasonable price eventually. Forget that you will get a like-new Stanley 51/52 shoot board and plane for $25, I said reasonable. There are too many people like me surfing ebay on a daily basis for killer deals to happen very often.
Don't get auction fever
It's fun and exciting, especially at first, but it is easy to get caught up in the heat of the moment and overspend. Set a limit and stick to it. See #2 above!
Consider using a sniping service
You set your upper limit and the service will do the bidding for you and guarantee that you will buy only if the price is at or below your maximum. This also allows you to bid on items that close at an inconvenient time, or if you simply get busy and forget. Another advantage of a sniping service is that it bids for you in the last 5 to 10 seconds of the auction so you don't tip your hand. The one I use is esnipe.com , and their rates are pretty reasonable: 1% of the purchase price in the range I usually spend (sliding scale). There are free sniping services, but you get what you pay for.
What is it worth?
In the advanced search section you can find out what similar tools sold for recently by looking at "Completed Listings". There are two cautions I would make: First, make sure you are comparing "apples to apples". For example, you should compare a type 12 Stanley #4 with another similar plane, not a type 2 (collector item) or a type 19 (late model piece of junk). Secondly, take the information with a grain of salt. There may have been two people that just had to have a dirt common plane and got into a bidding war, or a bidder was a complete rookie and paid way more than it was worth. Look at several listings and throw out any very high or very low prices.
Type studies-aka homework
It looks easy to buy a tool on ebay, and frankly, it is. It may even be too easy because you will be tempted to pull the trigger prematurely. You have to do your homework before you can buy with confidence. There are type studies on the internet that can be a huge help in determining the relative value and usefulness of the tools you see on ebay. For Stanley planes, three good references are:
My personal bias is Stanley bench planes from about 1907 to the end of WWII (1945). Older planes may serve you well, but in 1907 Stanley introduced the frog adjusting screw on its' common (non-Bedrock) line of planes. After WWII the quality of all hand tools began to decline. Other brands have their own type studies, but are harder to find on the internet.
Beware the Frankenplane
One reason to be familiar with type studies is that you have a better chance to spot a plane that someone has mixed various parts together to make a somewhat complete plane. Often this is fine, such as an incorrect blade when you are planning on replacing the blade anyway. However, sometimes it is unusable and it will always affect the value negatively. This may not be much of a problem if your intention is simply a user, be aware that selling it down the road may not be easy. You should always expect to pay less for a Frankenplane that a complete, correct one.
It doesn't have to be Stanley. My personal planes were all Ohio Tool for many years because their high quality blades didn't need to be upgraded. Information on brands other than Stanley is a little harder to come by, so you may have a more difficult time determining value. However, there are many competitors to Stanley that rate a look such as Vaughan & Bushnell (V. & B.), Ohio Tool, Millers Falls, Sargent, Union and Record. NOT O.K. are: Fulton, Shelton, Buck Bros., Wilton, "Made in USA, anything Handyman, and no name. There are store brands like Craftsman, Wards Master, Winchester, Keen Kutter and others that were made by one of the name brands and are fine. Some, like Winchester and Keen Kutter have their own following and you will be competing against collectors, while Craftsman and Wards Master are looked upon as second rate but they are not. Essentially all current production is pretty sad by these old name companies, but if it was made prior to WWII, it is probably fine.
Broken or repaired castings
Just say "No". There are way too many planes out there that have no major issues. The repair may be fine and cause no problems in a user, but until you get some experience, avoid repaired planes.
It is tempting to buy an incomplete plane thinking that you'll be able to pick up the parts you need in another auction. This can be done, and I've done it, but usually you will spend more collecting all of the needed parts than if you had just picked up a complete plane in the first place. Some parts are commonly missing, like the blade set for a #46. You will pay more for the blade set than you did for the plane body. So, you are better off paying $150 for a complete #46 with blades than $75 for one without and finding that you have to shell out $125 for the blade set. There are a few parts available as N.O.S., or current production will fit older planes, like the blades from a 71 router plane are still available from Stanley and will fit all 71 and 71 1/4 planes. There are also a few companies that make new replacement parts for commonly missing or broken parts for some desirable planes. Two sources that I've done business with are: St. James Bay Tool Co. (stjamesbaytoolco.com) and Jim Reed (jimreed2160 is his ebay handle). There are also dealers that have parts for planes, but these tend to be pricey.
The blade-MIA or ground to a nub
It is very common to get a better deal on a plane because the blade and/or chip breaker are missing or the blade has been ground down so far that it is useless. It's O.K. to purchase a plane like that IF you are planning on replacing the original blade with an aftermarket blade/chip breaker anyway. Figure on the additional costs involved; blade $25 to $60, chip breaker about the same, and don't forget the screw. Thus, it isn't impossible that your dirt cheap #5 could end up setting you back an extra $100 by the time you get done turbo charging it. It could be significantly higher if it is an unusual or exotic plane, although Ron Hock will make one-off blades for fairly reasonable cost (at least he would the last time I talked to him).
Wood doesn't lie
Be cautious about planes that have badly broken or missing totes and knobs. A crack down near the bottom of the tote (rear handle) is quite common and doesn't necessarily mean a plane has been abused, but if the wood is in really bad shape or missing entirely, it probably has been. Bad wood is frequently an indication that the plane has been abused and there will likely be hard to spot problems in the metal parts, like fine cracks around the mouth that are hidden by patina and won't show up until you start to clean it up. Look elsewhere.
Multiple items in an auction
Be suspicious if more than one plane is included in an auction. One or more may be broken, incomplete, or simply not worth buying. Do you really want both of the planes in the auction? There are times when a matching pair are worth more than you'd expect to pay in two individual auctions, like a matched pair of tongue and groove planes, but two common planes like a #220 offered up with a #5 is not worth a premium.
Incorrect or incomplete descriptions
Be suspicious of these. The seller may indeed be aware of exactly what he has, and is hiding behind phrases like "I'm not an expert" or "I'll let the picture do the talking", when in fact he only shows the unbroken side in the photo. Also be cautions of planes offered with only a few photos or are poor or out of focus. He may know exactly what he is doing.
You will typically pay more for these, but you can be more certain that it will be complete, correct and usable. There are vendors that specialize in restored planes or planes that are already in usable condition and they will stand behind what they sell. Beware of the plane that is too shiny or the Japanning is too complete, especially if the seller claims to have no knowledge of tools or if the great condition is offered up with no explanation. They have possibly been cosmetically "improved" to hide defects that may render the tool unusable.
Don't buy rust
There is no point. I'm not talking about a little "patina" or discoloration; that's expected and hard to avoid. I'm talking about real cancer. There are many planes available in good shape. See #2 above.
Buy from reputable sellers
These people know tools and don't buy junk themselves, so they don't have any to sell. As I mentioned, sellers can try to hide behind a phrase like "I'm no expert" and pass the responsibility of determining the usability of the plane on to you. Until you get comfortable that you can recognize value, stay with sellers that do claim to know a little and attempt to show and tell you about defects. These sellers are in business to stay and depend on positive feedback ratings, so they will disclose any problems with their planes that they are aware of , and will work with you if you discover something they didn't' notice.
The picture paradox
Part 1: As the number of high quality pictures increases, your confidence to buy the plane increases in direct proportion. However, the price also goes up because everyone else also has this increased confidence. Nothing beats a good set of clear pictures showing all of the important details for knowing you are actually getting what you think you are. Conversely, if there are no, or only a few fuzzy pictures, it is best to move on. Sometimes it is possible to ask the seller to post more photos, but generally it's more trouble that it's worth unless it is a particularly desirable plane.
Part 2: for gamblers only; fuzzy or poor quality photos seriously reduce the number of bidders, and I've gotten good deals on desirable planes, but I've also gotten burned. Be prepared for disappointment and don't ever bid more than you can afford to lose.