What I want to know is has anybody had any good luck with used graphics cards and if so where's the best place to look? Ebay? Amazon? I'm not looking for anything highend or even midgrade. I was looking at a 6670 or r7 240 caliber card. I just built a spare parts computer for a friend so he can play gta4 with mods.
ebay - look at asus/gigabyte products 7970-290x ask seller if he registered them. If he did then don't buy it. If he didn't even if card has issues you can register it yourself and rma. Obviously if asked where did you buy it from rma tell them on newegg. (7970's warranty support should end soon tho) Its best if you break your 7970 now and rma (in asus - if they won't find any 7970 in stock they will hold it for aprox. 2 weeks then will offer you new ones - 280x (decline, tell them that your 7970 was better than 280x in boinc computation or bitcoin - and if they offered you 290x you would consider it - and would most likely buy 2nd one to swap out your other 7970's) again they will take a week or so - either they will tell you that they cannot rma your card, offer 280x, or bingo 290x for free :)
Vanilla gta4 isn't exactly lightweight. Add mods to it and it will be even more demanding. I don't think cards of that level will deliver an enjoyable experience (talking about the 6670, 240 and the likes).
You could also try looking on r/hardwareswap on Reddit. They usually have a decent-good deal once in a while. Buyer beware, though. Usually an unreal deal is, probably, unreal.
Traded a dude on CL my 7950 vapor-x + $150 for his reference gtx 780. Have had zero issues with this card so far.