A hypothetical example might help to illustrate things:
SCENARIO 1:
Let's suppose that a secondhand dealer clears the house and spots a good quality antique table which he knows will fetch at least �300 in a general furniture auction. Some dealers might not even mention it and just pay a flat fee of, say, a couple of hundred quid, for clearing the entire house (which has other contents worth several thousand pounds at auction). Other dealers might want to show how 'honest' they are and say something like, "That's a nice piece. I can't really buy that with the general house clearance. I'll tell you what, I'll give you fifty quid for it".
SCENARIO 2:
An auctioneer clears the house. He's offered to do it for free but will charge 20% + VAT for his commission on sales. He spots the same table and, because he conducts the same auctions which the secondhand dealer attends, he also knows that the table will make �300 in a general auction. However, he also knows that he's got a specialist sale, for this type of furniture, scheduled in 6 weeks time. He clears the house and sells most of the items at a general sale. He keeps the table back for the specialist sale, where it makes �400. He charges �94 in commission (including the VAT) but that still leaves the vendor with �306.
Which would your friend rather have? �50 or �306? Both the secondhand dealer and the auctioneer want to maximize their profit. The only way the secondhand dealer can do this is to work 'against' your friend (i.e. to pay very low prices). The only way the auctioneer can do this is to work 'with' your friend (i.e. it's in both persons' interests that the auctioneer should get the maximum bid).
Go to an auctioneer, for house clearances, every time.
Chris
(PS: I was typing while Sandbach99 was posting. It's good to see that we're in broad agreement!)