Someone is offering to sell me 10x gtx1080ti here at 200$ per card, is this a good buy? Those cards are used and I'm suspecting it's been used for mining too, I mean who would buy 10x gtx1080ti for gaming or video editing ?
$ 200 is very cheap even for a broken card.
50 Mh/second this is no longer true at the current DAG file size, but you get 40-45MH depending on the choice of the power consumption mode.
The seller either does not know the market prices or is a cheat.