I find it hard to believe all these other sites -- FB, IG, Whatnot, Redditt, Etsy, etc. -- have folks who own high-end coins AND others willing to buy off the sites. I get Ebay -- even with all the unscrupulous frauds -- doing big coin business because it was a legit auction place dating back to the dawn of the Internet in the late-1990's. Surprised these others do business in any coins where serious $$$ are at risk.
I have seen high end coins offered on IG and facebook. Quite a few sell (nowhere near the quantity of eBay but still a decent amount). Whatnot, from what I hear, has a lot of activity too and some dealers sell a ton there (even some bigger names like Trader Bea and Witter).