if I were selling you a video card and you took off the stock cooler, repasted it, put on aftermarket cooling , then over clocked it, and it failed I would tell you too damn bad..
if I sold you one and it did not work then we would both know that in 15 min tops!
that's akin to you buying a car that registers 150mph and you wreck it trying to get it to that speed then ask for a refund as if it were defective because you crashed it before it went tha fast... I mean  come on man... doing all that would void any manufacures warrenty  would would you espect a private party to cover that...