First of all, this is not exactly the place to sell or buy anything.
You should try using the usual platforms such as ebay.
To be honest my 980 ti preforms way better then my 1080
If this is true then rather keep the 980ti and send back the 1080 then.
From the benchmarks i found on these two cards, the 1080 should be at least slightly better then the 980ti.
So your problem is more that you allready own a very good card and replaced it with another card that is only slightly better.
Frankly, if i had a 980ti i wouldn't replace it anytime soon.
Games are optimized to run on 5 year old console hardware or mid-range PCs.