A few months ago I bought an 8600GT 512 on eBay for 50usd to replace a dying 7600GT because it seemed to be an amazing deal. Since then I've had the suspicion that my card is not performing up to par. The suspicion was confirmed recently when a friend who always runs games better (often twice as good ) than me with his comparable 8600 system found out that he actually had an 8500. His slower system was completely stomping my faster one. A GPU-Z comparison showed that his card was completely superior in all aspects except in clock speeds. I looked up a spec sheet for mine and found out that my cards GPU-Z report showed exactly half in all specifications. I gave my card a good inspection (there was no sign of a sub vendor, only an nVidia sticker on the fan) and with a few Google searches found that my card is completely identical to the 1gb version and looks nothing like the 512 or 256 versions. And not only that, fitting with the half of everything theme my card is missing 4 ram chips out of its 8 slots for them while all the pictures of the 1gb version used all 8 slots and all 512 pictures had only 4 slots to begin with. I popped my old 7600GT in (I had never used it with my new cpu) and fought through the slew of graphical errors to try it in a game of Left 4 Dead to compare performance. Surprisingly, the 7600GT increased my performance in the game at the same graphical settings almost two-fold.
The guy probably just sold you a bum card, I'm not sure what the price was when you bought it, but a 9400 GT or similar is only around $45 new on Newegg. Never trust people on eBay until you have a reason to.