Go through this link which has some information regarding Contracts and Agreements. Try this out, May be it helps you
Legal Zoom may help. If the web application is already made or completed you just need something like - "This website is being sold BY, TO and is provided 'as is' - the seller provides no warranty or accepts any liability for the web application after the sale."
If it's already bought you could probably use a legal document for selling a car ;) - just something that says you except $X for it and assume 0 responsibility for it after the sale.