Amazon auctions: ok, so everyone knows Amazon, but did you know they do auctions too? This link takes you directly to the computers and software section of their auction site.
Auction Air: Auction air isn't specifically geared to computer or tech products but they occasionally have some. They use a reverse auction system - see site for details.
Auctionzone: Smaller UK online auction site. Some computer equipment always up for grabs. Not so many bidders so you could grab a bargain.
AVABID: Small UK auction website offering computer equipment, hardware, and software. There always seems to be a fair bit of computer kit, and you can pay a small fee to transfer your ratings from other auction sites.
CQout: Large UK auction websites. Lots of computer equipment always up for auction.
Ebay: Huge global online auction house. Ebay has changed the way the world shops. Thousands of computers, peripherals, and software items on at any given time.
Ebid: Long-established and large UK online auction site. Offers zero fee auctions for sellers.
Get Bidding: UK auction site with small selection of computer equipment.
Morgan Computer Auctions: Hundreds of computers and pieces of computer equipment for sale from £1 with no reserve. If you're looking for cheap computer kit then check Morgan out - long established UK high street computer retailer.
QXL: QXL was the pioneer of the UK auction market. Long established with lots of items up for sale.