It's not too good to be true. Cryptocurrency miners (mostly Ethereum but also Zcash) have been buying the mid range GPUs like crazy. Buying out all the stock and driving prices through the roof. Due to memory design. The GTX 1070 is better for mining than the GTX 1080.
Cash out while you can. This won't last. Just a short while ago you could practically have sold a Radeon Rx 470, 480, 570 or 580 and bought a GeForce GTX 1080. As Ethereum prices have started dropping and work difficulty has increased, a lot. Interest has started to wane.
Edit:
Looking at SOLD auctions on eBay. They are going for about $425 used on average including shipping. After eBay Fees (10%), Paypal Fees (2.9%) and Shipping fees ($10 to $20). I would...