No offense intended but that cpu has depriciated since it is older. 12gb of ddr3 doesnt go for much money anymore and nor do older motherboards. The 1070ti has great performance, however it is a generation old and they have decreased in price sharply in the past 6 months. Your case, psu and drives total no more than $150 usd. I agree with sr71 blackbird.
Are you buying or selling? If you are buying in hurry and haven't done much research, sure someone would be happy to pocket your $800. But if you are selling and you need money in hurry, you'd be lucky to get $500. With RTX2060 going for $350, the is n way I'd pay more than $250 for a used GTX1070ti. And then the other stuff is worth $125 tops, so I'd low ball you at $375.
This how markets work. And someone who is reseller would obviously try to get it for less and sell it for more and pocket the profits.