Do keep an eye on the history of the domain you are selling/buying, the value of being blacklisted, or known for spam, and negative things like those might have a direct result on the quality and the value of a domain on sale.
A simple and short, well established, and clean domain name with a good online exposure is worth more than a domain called 123buymylaptop123 registered 3 months ago (random example).
There are also quite a few sites online that scan the domains and give a virtual value to them, marketplaces, sitepoint.com, and of course domain forums (maybe even this one!) You could also for fun check the domain on dnscoop.com.