Considering that rig new would cost about $6500 in parts, the 1950X cpu alone costs $950, and figure $350 for the mobo, and about $750 each for the 6 1080TI boards ($4500 for all 6), and at least another $500 for the PSUs, RAM, SSD and cooler, I very much doubt the seller would let it go for $3500 the poster above me quoted you.

Even if the seller is desperate he can part it out and sell it off to gamers and still fetch $6k plus for the components separately, so I think that is about the price you will be looking at. Anyway, that seems to be too way much rig for general mining and I think you would be very hard pressed to earn your money back in any reasonable time-frame. The great GPU sell-off has yet to begin, wait another month or two and I think even at new prices you would get a much better bargain.