Just about everything for auction from a name we all know.

This is an automated site with a good interface. You can put up a tender request for required goods or stock. Full history of your buying and selling. Ability to re-auction items. Ability to upload a large list of items from your own database. Sellers profile.

This is a fully automated site designed around street names.With these main categories: Books - Business - Collectors - Computers - Electrical - Entertainment - Games - Home & Family - Leisure - Miscellaneous - Toys and Travel.

If you've not heard of ebay there's something wrong! The largest on-line auction in the world with over 1,600 list categories and over 250,000 items added every day.
Most of that is American, of course, but the UK side of th site is very good also.

fsauctions (Freeserve, Auction Universe) is a fully fledged and established auction site with too many categories to list:

Loot is another of the very well known, and big, sites. The main part of the site is for classified ads, but the auction side is big too. The items are not split into categories but require you to run a search for what you want, so if you are just browsing you are stuck with a long alphabetical list of the goods. Personally, I prefer the categorised system of organising what's available.

Morgan have an automated interface and a fairly short list of computer hardware for sale. The site includes some nice features including a statistics page giving basic figures for the number of auctions, bids and registered users etc. They do not categorise the goods but do let you view the results of recent auctions which will give you a good feel for the prices that the items are fetching.

The largest of the European auctions with a very respectable range of items for sale. One of the main features of QXL is that they list items for sale themselves.
These are their main categories: Arts & Collectibles - Cars & Transport - Computing - Electronics & Cameras - Entertainment - Good Causes - Home & Garden - The Travel Shop - Sport & Fitness

Needless to say, like all things Yahoo! the auction is big. With thousands of items for sale.
These are their main categories: Antiques & Collectibles - Arts & Entertainment - Business & Office - Clothing & Accessories - Computers - Electronics & Cameras - Home & Garden - Sport & Recreation - Toys & Games - Travel and Transportation - Other Goods & Services.