Domain age
Moz rank, Page authority and domain authority
info here and
here
Make sure the site your buying from is actually indexed well in google.
Number of other outbound links from the page that your link is going on.
Number of external link pointing to the site that you are buying from, and the quality of those links.
Overall quality of the site, look at it and ask yourself would you use it.
Someone with more experience in this could prob give you a much better answer that me however.
