I don't think they are barred from sale, but I do think that if you're selling secondhand CPUs on Newegg, the used nature of the hardware should be prominently stated. That is, for customers who are still willing to risk their money on Newegg.
With those caveats it's a great deal for something like a build box. You could put this into an existing ATX case with $1000 worth of RAM (that you may already own?) for less than the price of a new Threadripper CPU.
It’s become a marketplace ever since it was bought out, so lots of sellers of varying qualities.
As long as you always filter by “sold and shipped by Newegg” you should be fine.
Who are the recommended vendor for purchasing PC parts these days then? That is, who (if anybody) fills New Egg's previous niche? I've actually just bought a bunch of stuff from New Egg, after not doing any PC building for 15+ years, and didn't initially realized how much they had switched to the "marketplace" model.
With those caveats it's a great deal for something like a build box. You could put this into an existing ATX case with $1000 worth of RAM (that you may already own?) for less than the price of a new Threadripper CPU.