Craigslist is where I have had the most luck. You can usually get deals there easy and people selling on craigslist probably don't know what bitcoins are so they aren't jacking the price up
Decent cards are often sold due to upgrades, so they are probably good. However, many cards are also sold because they don't function reliably [they may pass Furmark and other tests, but still fail in ways that annoy the hell out of you]; these are often cards that have suffered thermal stress, over voltage, whatever, usually as a result of gamer overclocking attempts. Your risk to take. I saw a post a couple of months ago that indicated that up to 40% of the 5970s coming off of Ebay were damaged in some way [whether they remained useful for mining when treated gently I do not know].
Good luck and buy used hardware carefully.
Agreed. In the past when I've bought used equipment I've usually made a judgement based on the people. It sounds worse than it is. What shape is their house in. Is their computer covered in dust. Are they just some teenage kid who OCed his stuff like crazy. Etc.
The first time bought something off Ebay (LONG time ago) I got burned. Picked up a set of 3 hard drives and 1 of the 3 worked. Lesson learned. Buy new when you can.