Is buying and selling text links good, bad, or somewhere in-between, in terms of SEO? I believe that buying and selling ADVERTISING, in whatever form that takes (banner ads, text links, etc) is a normal part of doing business, and should never be penalized by search engines. It is only when you get into gray areas, such as buying or selling PR (or blatantly advertising that you are doing so), that may get a penalty or filter slapped on you. After all, Google owns the rights to PR, therefore they can get antsy when someone tries to profit from it. Nevertheless, my personal belief is that if you are purchasing advertising in the form of text links (and not blatantly purchasing PR), you should have no problems with penalties. And of course, advertising can bring in extra traffic, and possibly better search engine rankings.
Some places to buy advertising are:
Make sure you do your homework and feel comfortable with the site that you are advertising on. This is your reputation and possibly your future rankings on the line.