Buyer's Premiums is something many auctions houses started using to up their income from a sale. Traditionally, they got a percentage cut from the seller (as it should be IMO). However, as auctioneer's starting competing to get auction sales, they started offering sellers a lower percentage and then tacking it onto the buyers. There are arguments for both sides of the issue but I think it boils down to greed. Auctioneers were making good money on just the selling commissions.
The way it works, if there is a buyer's premium, it will be listed on the sale bill or announced at the auction. If you buy an item for $100 and there is a buyer's premium of 15% you will end up paying $100 + Sales tax + 15% of that total... yep, you even pay the buyer's premium on the sales tax so the auction company is making money on the tax you pay! (Something wrong with that). The buyer's premium goes to the auction company, not the seller who is still paying a commission.
So your $100 purchase + $7.00 sales tax = $107. $107 x 15% = $16.05. $16.05 +$107 =
$123.05 Your final cost.
Edited to correct math screwup...