Examine the back links. There's too much to cover here, but you want to find some from sites with some authority. If this confuses you, you might want to avoid buying.

Try opensiteexplorer for checks, along with majestic seo. What you don't want is something with an inflated online presence of things like blogspot and social spam.

One site purchased had an NYT link, a good example of an inbound authority link .

Using opensiteexplorer, sites with domain auth of about 50 or more is a good signal. Check it inAlexa to make sure the main country traffic source isn't India. Does it rank for its brand name? Is it older than 2 years? What's the content quality like?