It is hard to tell but for those prices (I think they are all to high except maybe the last one which does not list what Radeon GPU it has) you could build your own and get warranty on all the parts. First one would game best because of the best GPU but CPU is not so great any more.
The only you have listed I would even considered is this one (unless you can get one of the others for like $200) http://raleigh.craigslist.org/sys/3961507982.html If I had to choose I would call this person and offer $275 for it, bargain with him from there. If I had to pay more than about $325, I would simply build my own.
HD7770 is a OK card with relatively modern CPU but only thing there is the ULTRA PSU which is not the best but neither is it the worst. I guess it is the only one I would consider buying for $500 as it comes with Win 7 as well.
For instance check this thread for a $500 build that's pretty darn good, all new! If you have a case, you could upgrade the processor a little even.
I would never, ever spend $500 for system if it was used. They simply are not worth it, I don't care what it is. http://www.tomshardware.com/answers/id-1749032/500-doll...