$425 is a fair price for a GTX 580, 3GB or otherwise, but don't expect to be able to buy this card and flip it for a profit. This is pretty easy to determine by going to ebay and looking up completed listings for GTX 580. To make enough to make the flip worth while after ebay/paypal fees, you need to either have a special edition of the card (watercooled, Classified, etc) or be willing to ship internationally (which, after 10 yrs on ebay I just don't do - not even Canada). IMO, it sounds like too much of a pain to buy it with the intention of flipping, but it's a decent price if you just want a GTX 580.
edit: FWIW, I sold mine and I'm probably going to pick up a 7970 in Jan.