There's a deal (or was the other day) for a 2700 non X for $150 new on Newegg.You're always entitled to ask what you want for your video cards. That doesn't mean people are going to pay that for them. I get it that sometimes it only takes 1 buyer, but most people aren't going to wait weeks/months to sell their old stuff waiting for that one buyer who may never come.
$100 is on the low side of what I'd consider the 1060 6GB to be worth now. Realistically $115-140 depending on the condition and model. I wouldn't have been upset at a $100 "lowball" though.
In brentsg's case, I think I remember his cards and thinking, "There's no way that card is worth that." But like he says, it only takes 1 buyer. I wouldn't have written PM's to him about "joke" pricing though. Either make a reasonable offer or just move on to the next card. No sense in flaming a guy trying to get rid of his stuff.
As for the Radeon VII...I bought one for $575 in the forums maybe a month after release. I saw one go for $450 shipped maybe 2 weeks ago. Hard to say what a true value is for that card. It's marginally faster than the 5700XT for gaming, but I don't think it was every really meant to be a gaming card. It was just a workstation card repurposed for gaming.
Last gen stuff is always a tough sell for big money absent a market aberration. Look at the 2700X now in the FS/FT. Saw a few for $185 shipped with the Prism cooler. Bang for your buck, that's a kick ass chip for $185 (that's like $12 a thread! ).