As a part of marketing and SEO tactic webmasters go for buying text links, some for improving their PR, others for getting traffic and some just for publicity.
When buying links, you need to check for what benefit you will get from that link, will it be able to bring targeted traffic ?
If you get a good traffic through that link, consider your investment has paid for itself.
While buying text links you should considered following thing :

Page Rank: A link from a higher page rank site will give you better positioning and rank, though a link from a higher page ranked sites will be costly. Always try to have your link at high PR site.

Alexa Ranking: Alexa.com provides a rough estimate of the traffic that the website gets. Note that Alexa reports for the whole website, including the subdomain names etc. But still an Alexa ranking less than 100,000 is what you should be check for the linking site as this may ensure for the traffic.

Relevancy: You should always try to purchase links from sites or linking pages that provide content that is related to your site or page content. - You will get the most value from Google when the linking page or linking website provides content that related to your website content.

Visibility : Always check for the placement of your link. The link has to placed in a visible part of the website and not at much deeper page.

Number of Links: Your link should not be placed at a pages having lot of out-going links. This will diminish the value of the Page rank. If it has more than 100 outbound links, more chances are that your site link will lose its link value.

Check for nofallow tag : Your linking site should not be having the rel=”nofollow” tags in the link, which denotes that the search engines (at least Google) will not count the outbound link as valuable. Some webmasters do this blackhat tactic for preserving their PR.