When purchasing a spot on another site to put your link, it is best to link with a site that has similar content. When purchasing a text link the text that is in the link that points to your site should have the same major keyword as the page that it is pointing to on your site is optimized for. The longer you can keep the link up and paid for the higher the value that is placed upon it by major search engines.

The major search engines don't want to give sites that paid for their links on other sites the same high ranking as an editorial link because they think this might lead to a site with inferior or irrelevant content. They prefer editorial links where the link is placed there by the webmaster at no charge because the information is relevant and right on target for what their site is talking about.

If the search engine's crawlbot computer program sees more relevant content from a paid link then should it penalize? As of this writing the traffic that goes to the site that you paid to have your link put on is the single most important factor in you ranking high with major engines, according to crawlbot (SEO) experts. This crawlbot information is for future reference when you go to purchase a link spot, because they are currently working on the crawlbots to make them better able to tell which links are paid or editorial and what the content linking is.

The moral of the story would be just consider raw traffic from the site as the major factor in the purchasing decision and if there is a boost in the engines ranking that is just gravy on top. If the crawlbots attack paid links with new programming then there is no gravy but you still get the meat and potatoes from traffic coming in day and night directly from the site.

When purchasing space it is good to check the age of the site by 'whois' the older their site is the higher your rank will be by linking to them. Your link placed higher on the page will have a higher ranking benefit than if placed lower or on the bottom of their site.

When buying links see if there are a lot of other out going links on the site, if there are, a search engine may penalize your site (Linkfarming). It is recommended to not buy links from a source where you cannot choose the site(s) your links will be on because of the previous mentioned reason.

Some search engine optimization (SEO) experts warn to not put the same exact keyword in all links if buying multiple links coming in at one time.

When selling link space on your site it is recommended to put rel='nofollow' in the link's attribute. The 'rel' attribute tells the forwarding relationship that the other site will have to your site. This way the search engines won't 'bleed' off your ranking to the other site(s) that bought your link space.

There is a great referral program with a link broker where you put a link on your site and receive 50% of profits from the link for the next 3 months and 10% for the next 9 months, for either if someone buys link space or sells space. In other words, any buying and selling activity that goes on after your site got clicked with this link gets credited to you. These links are hundreds and even thousands of dollars on a weekly or monthly basis!

Another option besides the link broker's referral program is you may want to buy and sell links for your own site through them.

This should give you a basic overview on buying and selling links to bring traffic and income to your site.