One of the easiest considerations to miss when you are in the midst of a link buying frenzy is whether the domain name that you are buying the link from is actually still operational. Many domains that are in fact expired still show up in Google so you may unwittingly fall for one of them.
Check the status of the domains in question yourself and if they are expired it may be best to avoid them and save yourself the money; they probably will not be of any use to you. Probably Yes, because in some cases, even if the domain has expired it could still be of some use to you if it has good backlinks. Whether a site is good or not is largely dependent on the quality as well as quantity of its backlinks. Both active and expired domains can have a favorable backlink profile so do not automatically discount a domain just because it has expired.
You should of course always check your links for page ranking and make sure it is the current page ranking and not a fake one.
http://www.digpagerank.com/ is a good site to check your current page ranking.
One of the most important factors that can affect the quality of a backlink is the number of outbound links. Generally speaking, a good quality backlink will have as few outbound links as possible. The total number of outbound links on a page will also have an impact on the value of a particular page.
You should also consider the possibility of buying links as a means of increasing traffic to your site as opposed to buying links for the purpose of increasing page ranking. This could potentially provide more direct benefits and even profits for you.
Some other things to keep in mind: contextual links are supposedly seen by Google as being more natural and therefore will prioritize these over other links. .gov or .edu links are not treated as special in any way by Google so consider carefully the benefits of hosting such links. Look for a site that has similar keywords to yours for a better ranking in Google search engine results page. Finally, it would be best to avoid purchasing a site wide text link if all the other outbound links on that site have nothing to do with the site in question.