This plugin by awpcp.com is designed to allow you to add a classifieds ad section to your blog.
You can use it to add pretty much any type of classified ads service that can run under a simplified structure taking some basic information from the user. The core plugin is free.
There are Premium Modules add-ons which cost, inc:
Category Icons Premium Module $10.99
Regions Control Module $21.99
Google Checkout Payment Module $39.00
Extra Fields Module $29.99
A Per Category Fee Structure Module which will allow admins to setup their fees on a per category basis is in the development stage.
This Plugin has it’s own website at: http://www.awpcp.com