I promised this a few days ago, but I pulled a muscle in my back and have been laid up for a few days. (Didn't even know I had any muscles to speak of in my back, but then that's probably part of the problem.)
Anyhoo, what follows is my attempt to share what I've learned about buying user planes off of eBay. Note that I'm no expert, so I hope others will chime in with their own tips and tricks.
[1] Decide what you're looking for and set up a search for it. This can be a bit of a pain until you get it down to a system:
First off, you can't just look at categories because there are two different categories for woodworking planes on eBay -- one in the collectibles area and another in the non-collectible tools area. Also, some sellers are clueless and put things in the weirdest places. (These often turn out to be the best deals, as those who go by categories miss these items completely.)
Second, if you search for "plane" you'll get a million irrelevant listings, so you have to narrow things down a bit. You'd be surprised, for example, just how many athletes there have been over the years named "Stanley" somethingerother. So you need to get around the memorabilia or you'll be looking at nothing but baseball cards.
To do this you need to use the "not" function in eBay's (really pathetic) search engine. To use the "not" function, you simply put in a minus sign followed immediately, (no space between), by the word you do not want the computer to look for.
Here is an example of one of my searches, set up to look for a Stanley #4:
stanley 4 plane -TOPPS -memorabilia -VHS -eagles -thermos -sweetheart -shoes -boots
Third, you should really set a limit on how much you are willing to spend for the item, but I'll cover that in detail below.
[2] After you have found a bunch of the plane you are looking for, go through the listings and see which ones look like they'd be good users:
Ugly is good -- we want to use this thing, not put it under glass in a museum.
Stay away from Sweethearts. They sell for way more money.
To a certain extent, rust is your friend. Provided the tool is not falling apart, a certain amount of rust is really no biggie.
Look for planes with most of the Japanning gone. Have you ever seen a car where the paint is basically intact but the body is completely rusted? (Very common in the North East.) If you see a tool with quite a bit of rust on the blade and such but the Japanning is intact, there is a chance that the paint is hiding a bunch of really hideous rust that is compromising the structural integrity of the tool.
Here is a really good tip: Look for planes where some previous owner has either engraved his name/initials on the body, drilled a hole in the sole to hang it on a nail, or some such thing. Purely cosmetic, but it drives the collectors away in droves.
Stay away from sellers who don't tell you up front how much it will cost to ship an item, either directly or by including a shipping calculator in the listing. IMHO there is little or no excuse for any eBay seller not to use the shipping calculator, because it costs them nothing. Keep in mind, however, that the shipping calculator allows sellers to add hidden fees onto the actual shipping cost. (I strongly suspect that many eBay sellers make more off of shipping than they do from the items they sell.)
[3] Use the "Watch This Item" function to save all the planes that look decent but not pretty. Wait until you have built up a small database of concluded auctions and then compare the actual selling prices to your assessments of the tools in question. Figure out what the price range is for a likely user in that category. Also figure out what the average shipping cost is for that item.
[4] Figure out the maximum you are willing to spend for a given item -- including shipping.
[5] Go back and run another search, but this time use the "Advanced Search" function so that you can search only for items that fall below your maximum purchase price less the average cost of shipping your item.
Example: You determine that you should be able to buy a decent user #81 3/4 for about $18, and the average cost for shipping that item is about $9. Run your search with a max price of $18, then look at each listing to be sure that the seller is not going to hit you for more than $9 to ship.
[6] Scalp!
In a rational market you would simply input your maximum bid as soon as you find the item and leave it at that. Unfortunately, eBay is anything but a rational market. The mentality of many folks who buy on eBay is similar to that of folks who are addicted to the TV shopping networks -- the mere fact that someone else wants something makes it more desirable. So, avoid bidding until you have to.
[7] Ask question of sellers, and if you can't tell what condition an item is in, ask for more pictures. If answers and/or better pictures are not forthcoming, walk away.
[8] Remember, there will be another one tomorrow. If you get out bid just walk away. If you get out bid repeatedly on the same model plane, then take another look at your analysis above and see if you are really in the ball park. Make adjustments if necessary, but don't be too quick to jack up your price just because you miss out a few times.
That's all I can think of at the moment. I hope somebody finds this useful. Also, I hope that some others who have bought planes (or whatever) off of eBay will chime in with their tips as well.