On ebay I buy from the clueless, since I know my material and they list the items cheap.
On forums I like to buy from the people who collect what I collect and are too lazy or busy to ebay the item for what it is worth (on ebay).
On newsgroups and email lists I like to snag things people run across (or no longer use) that they do not collect but do not want to throw it away.
Some thing has to be very rare and I need to want it very badly to even think about paying ebay market value for it. The fact is if you collect for a long period of time you tend to find the rarer things easier and cheaper.
And what does annoy me on ebay is people who yank cards out of complete machines but do you include the HD (for rare drivers) or Dongles/cables needed for the system to work. You get a nice rare working system and then make it worthless.
I think I fall into all of these categories from one transaction to the next. Sometimes I'm too busy/lazy to properly ascertain the functionality of an item, so I tend to price it low. Sometimes I unload things that I don't really collect, or have too many of. And, occaisionally, I'm even klewless about sum'n, but certain that somebody, somewhere wants it, so I price it low, and let the market decide it's value, if any.
Case in point: Right now, I'm unable to locate the st00pit video cable for an otherwise very nice CP/M system that I have, which includes software & manuals, and was known working when stored, but I can't check it out now, because I don't have time to sort out the RGB signals on both ends of the cable, one of which is an 8-pin DIN and the other a flat dual 8-pin connector, so it'll be eBayed as-is, prob'ly for a lot less than what it's worth.
On the flip side of the same coin, I just had delivered today, a 34 lb. box of vintage Telxon tablet PCs & gear that cost me $5.00 + shipping on eBay, prob'ly because other bidders didn't really know enough about it to decide if it's worth the five bucks. Ya win some, ya lose some...
--T