WP ZinText is an easy way to implement in-text advertising on your blog. Link ads are only displayed on single post pages, and you can set them to only display on posts older than X days.
Question / Problems with WP ZinText? Go Here: http://www.zintext.com
To use this plugin you will need a campaign ID from eBay Partner Network. For more information on ePN and what a campaign ID is go here: http://www.zintext.com/faq.php