It is absolutely not worth upgrading from a 970 to a 980 whatsoever. Marginal performance improvements at best, and overclocking your 970 can have it match the performance of a stock 980 on most applications. Also, selling your 970 would be a hassle and since it is used, you will not get close to what you paid originally for the card. Absolutely not a good idea to do that.
Honestly though, unless you want to play games at 4K, or game at 60 FPS on ultra detail at 1440P, you don't need two 970s either. I would save your money if I were you and put it towards another upgrade, maybe the CPU, motherboard, an SSD/an additional SSD, more RAM, a better cooling system, games/software, or save it for some other gift or aspect of your life.
At...