$300 is a no brainer buy. $350 is what I would consider fair value. $400 you are pushing it. Insanely a month ago some were going for $425, $450, $500 and yes even $550. No fraking clue what those noobs were thinking. No way it is gamers. $550. Lets see a 7970 or a 2 generation old 5970? Hmm. Not a card call for a gamer.
That's not the only models that happened to during the same period of time. 5870's and 5850's jumped $30-$50 above fair. 5850's were selling for like $190-$200 and 5870's were selling for $225-$240. Why people would be willing to pay high prices for second hand items amazes me. When I buy from ebay I'm selective and patient sticking to my values, looking to get at least one year of use out of a card so I pay it off and generate some profit.