Up until the 1960's Auction Houses only charged the seller(not the buyer) Then an auction house cottoned on to the idea of charging the buyer too.Other auction houses said they would not charge the buyer,but had all jumped on the bandwagon withing 2 years.
The commission (for Seller and Buyer) varies,but goes from 10% up to a whopping 25%.
The commission attracts VAT as well at the standard rate.
If the item doesn't sell you will only be charged a fee if you have set the reserve (in the auctioneers opinion) too high.
If you leave an unsold item with the auction house you might be charged a storage fee(this is at the auctioneers discretion).
Credit Card payments usually attract a charge too.
Can you be surprised that auctioneers are wealthy companies.
This is why many people sell on EBay,no fee for the buyer (apart from delivery/postage) and small percentage charge for the seller.
Not only that,but selling on EBay gives you an enormous buying audience over that of the auction house.