There are coins worth hundreds of thousands listed on eBay, and are legitimate. The problem is sellers have to mark things up 10% to cover fees. This isn’t a big issue with a $20 coin, but gets to quickly be an issue for higher end coins. I think a lot of dealers list them on eBay for marketing thinking smart collectors will go to their site and see it is cheaper. I do that myself. Surprisingly people pay eBay prices often, and it would be better for me and them to go to my website.
With that said agree with all others. eBay prices are going to be higher to cover the fees. People still buy at those prices so they are true comps, but still high. Other auction houses will have better info on higher end coins. Greysheet and auction houses don’t do much with coins under say atleast $100. eBay is the go to place for moderns and lower priced coins and that is your best bet. So really depends on what you want comps on.