From its content, traffic, PR, and the visual design of course. If i like it, i will think that the site is good
If the PR, content, traffic and the visual design of the page is good then the site is also good site.
If your sites contents is good, high PR,visual design of the page is good, used in h1 tags and alt tags then the site is so goods.
I think most important aspect is PR & traffic of the site . Other thing can also consider like design , site load time etc.
I would pay attention to who they link to and who is linking to them. Make sure they aren't linking to anyone you wouldn't link to. Plus all of the stuff above.
monitor it for a good amount of time and see if your interested in buying it if they'll provide traffic stats.