www.freeads.net This has a published printed equivalent and is free for private use.
www.freeadmart.co.uk This also has a published printed equivalent and is free for private use.
www.adtrader.co.uk This too has a published printed equivalent and is free for private use. I do believe that it reaches a wider national audience though.
www.friday-ad.co.uk Another one published in print as well as on-line and is also free for private use. When placing adverts, you have to post them one at a time to avoid charges.
www.free-ads.co.uk This is purely online as far as I know and I don't think it is that widely known yet.
www.exchangeandmart.co.uk I think everybody has heard of the printed version of this. However, it is only free if you are giving stuff away.
The Sun Newspaper also allows you to place a classified "For Sale" Ad on-line. I don't know if it goes in the paper as well. www.local.thesun.co.uk
www.gumtree.com or bournemouth.gumtree.com Both these sites are purely on-line based and cover a multitude of things from buying and selling and property rental to dating. I have added the Gum Tree site for my local town on this page. Check the site to see if your town has one. If not, your nearest major large city probably will have.
Many Gum Tree 'clones' seem to have sprung up, such as: www.bananabean.com and www.bananabean.com/bournemouth I will add more as I become aware of them.
www.greenmetropolis.com This is an excellent site for selling unwanted books and buying different ones. You can also help the environment and donate to charity at the same time. There are also links to some good book sites: Here.
www.parkers.co.uk Strictly speaking, this is not actually a selling site. It is a site to check the market value of a vehicle you may be buying or selling, based on its age and condition. This too has a printed version.
If you do want to buy or sell a car, then you could always try Auto Trader : www.autotrader.co.uk - once you have used Parkers to find the optimum buying or selling price.
I assume that almost everyone has heard of Amazon: www.amazon.com or www.amazon.co.uk and the auction site, E-Bay: www.ebay.com or www.ebay.co.uk another less successful auction site is QXL: www.qxl.co.uk NOT to be confused with: www.qxl.com which seems to be a VERY poor European subsitute. There is also: www.ebid.co.uk.
If you are looking to either let or rent property, then Here is a good starting point.