I think the biggest problem is that the GPU manufacturer will not know if the card has been used for mining or not.
You can easily say that you are a reseller of the cards and that you getting them RMA'd for your current customers.

Although I think that they will figure something out to add onto the newer gfx cards to check how many hours it has worked hard for ect.
Just like how manufacturers put into ssd's on what the power on time is, how many times it has been powered on, the amount of times its been written to ect. So I wouldnt imagine it would be hard to do.