Where in the world are you getting your video cards is the real question (send in a private message so we can keep it a secret XD )
for a 970 you should be able to get at least $200, maybe up to $230 if youre lucky.
A 980ti generally is ranked as slightly a better card than a GTX 1070. That being said you are still buying a used card and a older card which hold some risks. (I'm actually suprised, I just looked on ebay for 980ti's and they are dropping to around $300 which is amazing)