When an auction house adds a premium, a ALWAYS add that to my bid, and compare that with what I would pay in the wider market place.
For me anyway, a buyer's premium just makes it harder for an auctioneer to sell to me.
Every form of selling to a customer has it's 'on' costs:
For
ebay, it is Paypal and shipping costs,
For retail, it is profit margin to cover business operationg costs,
For auction, it is the same as retail,
For purchase from an anonymous seller, it is authentification risk.