"Stand by for the flood of (ab)used GPUs on ebay when the bubble bursts though..."
But that has already happened. The last couple of big BTC price drops (9k euro to 6k) resulted in similar drops in the price of alt coins, and the alt coin price stayed the same while BTC recovered.
So something like a 1060 was making about 2-3 euro a day now makes a bit under a euro.
Therefore there *should* be a flood of 1060s and 470/570 second hand cards.
With the exception of the fans, a second hand GTX mining card should be in better condition that one used for gaming. You can't fuck with the BIOS on it*, it'll generally have been run about 65% power at about 90% of it's max settings, and it will generally have been well ventilated. Gamers will often run a card to max conditions, in a closed case, with no regard for any stat other than the faps :)
I'd be wary of second hand ATI cards, since you can modify the BIOS on those without much hassle.
Either way, it's not going to make any difference to GFX prices.
* OK,you can, but you'd have to be pretty nuts to