Buying and selling are two different things where different strategies should be used!
But here is what I do when:
Buying: Look at every single aspect of the website and come up with a value of how much the website is worth in your eyes. Check traffic statistics, monthly revenue, how the site looks, what kind of content is on it, is it unique?, could you make money of it? Ask yourself lots of questions!
Selling: When selling a website you want to get every penny you can possibly get for it. Be sure to include graphs/statistics about your website and cover everything possible. Traffic Stats/Revenue/Backlinks/Pages Indexed/Domain Name/etc. Your best bet is to auction off the website as well as offer a BIN.