If you don't already know about E Bay, its an online auction site, but you can also put a price on items and offer them at that price. You just go to www.ebay.co.uk and sign up for an account.
If you want to buy and sell through E Bay, you should also set up a Paypal account, which safely transfers money between Paypal accounts, without the need for either party to know each others creditcard or account numbers.