What would be the best bang for the buck paired with the mbest 1440p 144hz monitor (supports freesync)? Probably buying by end of this week thanks to tariffs.
Primary games : D3, PoE, Overwatch, BO4, DotA 2.

How old is the used mining card? If it's been in use for over 6 months, I would personally stay clear from it. These cards are not made for 100% usage 24/7.
So it's more 1070 ti vs the Vega cards, and that is kinda a tough decision. On one hand the Nvidia card will outperform the Vega 64 slightly with better driver optimization even for a lower price, on the other hand you will miss out on the freesync feature.

Gonna submit my vote later.

No info on how long the 1080ti has been used, but he's selling 2 of them.
And that's kind've what I'm wondering is if the overall performance increase of the 1070ti would outweigh the freesync on the Vega cards if I'm not playing much new AAA titles. I also don't mind turning settings down a little to meet better framerates (high instead of ultra).

1080ti

Most intelligent/serious miners clock their cards to stay at a certain temp, usually around 70C with a cap on the power draw to stop them from wasting energy (my understanding). Rarely are they overclocked beyond GPU Boost 3. The only real worry is how hard the fans were worked during mining usage. I'd still bite on it. That price is great. Mining 1080 Ti are still usually around $500 on hardwareswap.

1080 ti easy
Only risk is if card was mining unstable for a long period of of time cause the guy didn't set it right.

Unless you want to 4x RX 470 8gb for the same price lol

I nulled but I was going to pick Vega 56 before just because it's the lowest costing. Though 50 bucks more for a 1070ti is nice and then 20 more for a 1080ti. I'd take the 1080ti. You'll probably be fine. If he is a serious miner then he is probably upgrading his cards to the next best thing.