Depending on what type of business / website it is, may be worth sticking an ad onto Daltons (
http://www.daltonsbusiness.com/buy/business-for-sale-listing). I've
brought 'real' businesses there in the past (ie not just website powered).
Other places that also get enquiries are Gumtree and eBay. Just list it right, be descriptive, and expect to get some time wasters. Again, I've
brought assets this way, as generally businesses listed on ebay and GT are sole traders, where as Daltons is more Ltd.
Should say that I've never sold a business by either of these methods, only
brought, and as a buyer, that's where I look.