I just look at my use of it and tend to buy used gpus when it comes to gameing, they tend to be one step down from the top line and the better brands.
my main game cards are new.
cards I buy just to muck about with I buy all used, I have all ways wanted a titan, I just bought one with water and the aircooler but for 1/3 the price of a new card alone.
I'll have it for a while, do some things with it and trade it off.
my working cards I buy new, teslas and quadros, the cost is huge but I need the 24/7/365 hardware/software support for those.
if I am looking into a card just after they are released, I will buy from a known source and will be picky from whom I buy and if none show up on the market, then I'll buy new.
I like to run multi card so it can save quite a bit buying used.
trading my used cards has worked out pretty well, not to long ago I traded my 4 gig 770, that I valued at 150, and 200 bucks for a pretty new 980, he wanted to raise a little cash so I got a current, very good card for 350.