I am pretty sure that the cards selling for $350 on ebay are plain 1080's and not 1080ti's.
If you find a ti for $350, let me know because I will buy it immediately.
The 1080's are $350 used which should be close to 2070 performance.
The 1080ti's are about $500 used, which should be close to 2080 performance.
Were looking at a $200 price difference for similar performance at 1440P (2070) or 4K (2080)