Here's a link to a very good article about buying sites or domains for SEO purposes
Tips for Buying Old SitesOne of the most important tips is actually a warning
I will warn you not to buy domains to re-purpose the link equity if they aren’t related to your website. Buying car websites and then pointing them to your dance websites is a practice Google frowns upon. The further you go, and the more it looks like you are trying to trick Google, the worse the penalty may be. If you buy lots of charity domains and repoint them at your commercial domains, you may find yourself the proud owner of a page rank zero website.
However, buying domains to increase search visibility is a very viable practice amongst established SEO's.
What do you think about this topic?
Buying Domains for SEO Purposes