This is what I figure too. I might have a price range in mind for a given grade, and the calculation most always includes eBay as a source, but I don't rattle off sources and prices. I just stick to my range and negotiate into it or walk.
why? Because I hate when dealers say to me, "I can get more on eBay!" To which I reply, "Yeah, but this is I'm standing here with cash bay!" No return because you see me walk away happy, no non-payment, cash in hand. That's worth something too. Reference eBay all you want, but I can't stand when either side uses it as an exact negotiating tool. (Exception -- if both sides take a "blended rate" on a highly liquid book such as ASM 300 where there's lots of current transactions for most any given grade. I think that's mutually fair to do together. But most gold and early silver, the N of transactions is not big enough to set a price in stone. A floor maybe, but not a final price.)