As Chris indicates,Auction Houses used only (when I was younger) to charge the Seller,the Buyer paid nothing.
Sadly quite a few years ago,one large auction house,I think it was Christie's,bought in a (variable) buyers premium.
ALL the other Auction Houses raised their hands in horror and said "We will NEVER charge the buyer".Of course when they saw the money that the Buyers premium made they soon jumped on the bandwagon!
So now at most Auctions not only does the Seller pay a fee,so does the Buyer!
Not only this but if the Buyer pays by Credit Card a further fee is involved.If the Seller sets the reserve too high (or what the Auction House thinks is too high)and the item doesn't sell because of this, they can charge the seller a fee for that too.If the item remains unsold and goes into the next Auction, some Auction Houses charge a storage fee too!
They can't lose can they?
This is why EBay is flourishing.Sellers only pay a fee dependant on the sold price,and the Buyer pays nothing but the end price (plus P&P)
It is is to be hope that is this recession the greedy Auction House will realise the error of their ways and at least lower their fees etc.