I agree with BigBANGtheory. 1080Ti will be stronger but on benchmarks the 980Ti's might be a tiny faster
I also agree about knowing the background of the cards. If you buy a good enough brand though off eBay, you have good comeback if it's screwed. I've always bought off ebay and met the person to collect. Don't trust the London based sellers that do not allow local collection! (or any that dont allow collection for that matter)
One guy I bought off even invited me into his house and demos the 780Ti in games and benchmarks. I did the same when I sold my 980Ti's, nothing to hide if they are in perfect working order.
It also helped my EVGA cards had extended warranty, were still in warranty and it could easily be transferred to a new buyer
I also agree about knowing the background of the cards. If you buy a good enough brand though off eBay, you have good comeback if it's screwed. I've always bought off ebay and met the person to collect. Don't trust the London based sellers that do not allow local collection! (or any that dont allow collection for that matter)
One guy I bought off even invited me into his house and demos the 780Ti in games and benchmarks. I did the same when I sold my 980Ti's, nothing to hide if they are in perfect working order.
It also helped my EVGA cards had extended warranty, were still in warranty and it could easily be transferred to a new buyer