if i buy a r9 290 or some other gpu on craigslist, whats the best way to ensure that everything is in complete working order. what kind of tests should i do what should i look for yada yada yada blah blah blah...
FurMark for 20 minutes and keep an eye out for artifacts. Be sure to crank the MSAA to ensure maximum GPU memory usage to test the memory as well.
Buy something that still has warranty on it and comes with the origianl invoice. I recently RMA'd a card that I bought used on Ebay without a problem. All you need is the original invoice.