The two cards perform practically the same, outside of the 2080 having 3GB of VRAM less and the 10-series being terrible at realtime ray tracing. So, it pretty much comes down to:

If you play games that take advantage of ray tracing (and you want that), then the 2080 it is, but if you want more VRAM, then the 1080 Ti is. However, if you're hesitant to buy used, then the only option is to buy new.

On the other hand, if you want both ray tracing and 11GB of VRAM, and 20-30% more performance, then the 2080 Ti is sitting there just waiting to empty your bank account.