Just now, VengefulCelery said:
I am interested in building it myself, I was just thinking "if this is a good deal, might as well". I'm not insistent on building it myself. I think I was just inflating the value of the 1080ti.
Performance wise the 1080ti is roughly equal to the RTX2080 which starts at about $700. So for a 2 year old used card you would want it to be cheaper than that... Though it's rumoured that in the next few weeks price will drop as Nvidia releases some new Super cards (rumoured!). Also have new AMD Ryzen 3000 CPUs and AMD Navi GPUs releasing in 3 weeks time.
Since your computer is still decent I'd hold off a couple of weeks and wait to see the aforementioned products released and check out some independent reviews for how they compare.