Here are three items that are important to look into when deciding whether or not to buy a site on places like flippa :

1) PR ( higher PR, the value of a site is higher )

2) backlinks ( check a few of them to see if any have pr )

3) age ( the older the site, the more the value )

You can use a plugin for firefox to few these items : 

Quirk Searchstatus

I use the plugin all of the time:)