The length of time a cheque will take to 'clear' depends on the type of account you've paid it into. Banks make vast amounts of money from 'sitting on' funds while they're theoretically being cleared. With some 'premium' bank accounts, a cheque might be cleared on the same day. With, say, a Barclays 'basic' account it takes 5 working days (which, unless there are any bank holidays involved, really means a week).
If you've sold something on eBay and don't want to post it until the cheque has cleared, the only safe thing to do is to wait until the balance of your account (as shown at an ATM) is matched by the 'cleared funds' figure.
Unless a cheque has been properly endorsed with the number from a cheque guarantee card, it can be cancelled after you've deposited it but it would cost the person who wrote the cheque quite a lot to do so. (Typically between �20 and �50, depending upon their bank's rules).
Chris