I've also checked the Amazon prices for new and used cards, and set my "buy it now" suggestion based on those, in conjunction with the fact that it throws in the other hardware for free, and that it's already tested.
only rich kids get their parents to buy them a 1080ti for 1k on amazon, but anyone who has a single clue about mining knows the prices for these gears.
500$ for each card and giving the rest of the components for free is the average fair price as of today's market, you could get a bit higher than that in EU zone, i have bought a bunch of these for a client last moth for an average price of 450$ locally, so really 500$ is a perfect deal from a seller's stand point.
also it does NOT matter who built them or how they did it, your cards won't make 1 cent less if there were put here and installed by my grandmother.
the above rig was not built by my grandmother 
I don't intend to be mean , not trying to low ball "obviously i have no interest in these rigs", but i am just trying to help you out with some facts which you might not be aware of.
New cards on Amazon are $1,400 to $2,000. Used ones are about half that.
Amazon prices are "way-off", an old gtx960 going for about 400$ while RTX2060 selling for 350$ , RTX2080ti is priced at 1200$ >
https://www.amazon.com/GIGABYTE-GeForce-Graphic-GV-N208TGAMING-OC-11GC/dp/B07GJKR7RW/ref=sr_1_3?keywords=2080ti&qid=1555560726&s=gateway&sr=8-3this should give you an idea that a brand new 1080ti is no where close to 1400$, let alone 2000$.
good luck.