While mining may not be what Nvidia made them for, it's what most of them were sold for, so I think the warning on their likely condition still applies.
Beyond that, I'm pretty sure that the Techpowerup DB is wrong on the PCIe bandwidth. The first-hand reports I've seen online says the P106's are limited to PCIe 1.1:
$20 is a fair price for those that actually need exactly what the P106 provides, but the cheeky flipside argument is that having a near monopoly might make even the e-waste overpriced. :)