I'm not talking about the cream of the crop coins. I understand what you are talking about, but if you see a comparable coin on any of these sites buyers won't pay more just because it's sold by Heritage or Stacks. It's pretty easy to see each ones online auction listings. Yes, certain auctions (ie, Long Beach) bring the buyers with deep pockets. You usually don't see run of the mill coins in those auctions. I've bought coins from Heritage and GC auctions and I always consider the buyers premium.
Most people consider the buyers premium, most consignors at Heritage or Stacks are getting part of that money as well too. David Lawrence will probably start being a bigger player now too that they're using real photos and have no BP if that is important to people. There certainly is a price point where it no longer matters what venue it is and there are coins that will set a record in any venue, but that said the additional marketing does seem to have an impact in that middle ground.
Heh, a bit of luck on that estimate. But, as someone who has dabbled a lot recently on eBay, I figured the man didn't want to chance losing money on this lot if it didn't get the traffic he wanted.