Auctions with a "Buyer's Premium" are so weird.
'Yes I think this thing is worth $100 and I'll pay that for it.'
"Great, we'll be charging your account on file $120."
'But I said I'd only pay $100'
"Well, you're only paying $100 for the thing, but it's an additional 10% to buy it from us. Then of course you have sales tax on both the item and also on the service of us selling you the item."