The resale value of GTX 10 series video cards has plummeted in the last week or two, but I think that's due to the impending release of the next series from NVIDIA (probably will be announced at the end of the month, whether a paper launch or a real physical launch remains to be seen).
There are only rumors about the upcoming 2 different variants in the market. Nvidia Ampere and Turing. Allan Turing is a cryptanalyst so it would be easy to speculate that they are going to create two different cards in two different markets. Overtime, the 2nd hand GTX 10 series will flood in the market and for sure, like cryptocurrency, will be extreme volatile in price. I'm projecting the value of 1070's to fall below $250 given the current situation in the buy/sell community.