Amongst the most common appraisals I'm asked to give is for established online businesses. When selling a domain name, the business owner is often the best judge of the present value of the domain. Ask yourself these questions:
Many online businesses simply couldn't survive under a different domain name. That could be due to some characteristic of the name, e.g. very generic, or because of the established awareness of the name. In these cases, the domain name can be considered to "be" the business.
Under some circumstances, selling your domain name might be more profitable than the underlying business, and this would certainly be worth consideration.