Here's an easy experiment:
Call up Christie's, Sotheby's, Heritage, giant car auctions, etc, etc.
Tell them you have a collection worth around $25,000 and that you are interested in consigning. Ask them what percentage they will charge you.
You don't mention the buyers premium and see if they do.
Report back here what they say. For extra credit, if they tell you they charge 15% (or whatever) say "So if one of my items sells for $1,000 then I get $850 of that?" and see how they reply.
When I say that my competition LIED, I mean that they showed the consignor a printout showing my commission rate, and then said, "We only charge 15% for catalog sales and 25% for Internet sales, which is way less", and that IS a complete lie, because I charge WAY less than they do, when you compare what the buyer paid to what I (or they) pay the consignor.
If any of you (except for the trolls) want to say that they actually charge LESS than I do (because the buyer pays the BP, not the seller) then I will seriously consider adding buyers premiums, but lowering my commissions so I charge EXACTLY what I do now, which will then still be under what the others charge, but without any wacky argument that I don't charge less due to semantics.