I do not really know any sites that are for the selling of businesses. If it is an internet business then I would suggest a message board like this one. There are others though, that have many higher players running around. You could always try to contact other businesses that are similar to yours. They might be interested in buying you out.
If it is online business then why not try market place section of DigitalPoint? And if it is offline business then i think giving advertisement in local newspaper will be bettr.
If its an online business then you must have heard of Flippa or DP BST section but if it's offline business then you need to contact local buyers. For example, I am in India and by no means I can operate a business in London.
Is it a web based business or do you have a retail store and a "physical presense". If its web based there are a number of channels to resell it. If its a brick and mortar business you may need to find an agent to help you sell depending on how large (dollar wise) the business is.
great question would be good to know and maybee you will even find someone in this thread to buy your business