The non-availability of the RX 480 makes valuing older cards a bit more complex.Basing it on what is available with similar performance and price in new cards. I would take a 480 or 1060 over a 970 or 980. Especially a 970 with how reliant it is on drivers to function properly. Nvidia cards don't age well and DX12 is here now and more and more big titles are releasing that use it.
My sig is not updated, I only have one 980ti - sold one, I'd price a 980ti today at $300 given the 1070's price. I doubt I'll get much more than $400 CDN when I sell the remaining one. I sold the other one for $600 a couple weeks before 1080 released, that pays for half a Pascal Titan, lol. The new cards kill the value of the old pretty fast. 970/980 used aren't worth more than what I said, at least if I were buying one, which I wouldn't even at those prices.
I just had three bidders in 24 hours for my GTX 970 posted at $175, so clearly, there's demand for it at that price. You may not want a 970 at $100, but that's a good thing, because I can guarantee you no one in their right mind would sell you one for that much.