Buyers premiums are there to trick newbie buyers, but more importantly they are there to trick CONSIGNORS.
Say a person approaches an auction that tells them they charge 15%. That person reasonably assumes that if the buyer pays $1,000 for their poster, then they will receive $850 ($1,000 minus $150).
But in actually, when the buyer bids $1,000, they pay $1,000 plus $200 or so in buyers premiums, for a total of $1,200.
So that seller receives $850 out of $1,200, meaning they paid just under a 30% fee, nearly double what they THOUGHT they were paying.
All completely legal of course.