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.
What do you mean by GPU sell off? What part of my rig specs would be affected by this or how would it be beneficial to wait and get something else in a few months?
As mining becomes less profitable used cards will continue to be dumped on the market by people who can't afford to continue mining. This could be for many reasons (bought on credit, over-paid, less than expected returns, not profitable due to higher energy costs, etc). Whether you should wait or not is dependent on your attitude toward what you think will happen next, ultimately nobody has a crystal ball. Paying full price for a GPU rig right now is an objectively bad decision right now. You'd be better off either investing in an Asic that has been priced according to the expected return, or buying a used rig/cards. If you're dead-set on the GPU rig I can't stress enough not paying whatever that guy offered. It's extremely over-priced. Also if you need someone to host your rig, and can't do it on your own I'd be happy to help. Feel free to PM me.