When an ad is posted it will be placed at the top of the category selected and the top of the freshly added section on the homepage. As more ads are placed on the site your ad will go through the rotation. If you were not able to sell or trade your item you have the option of placing your ad back to the top of the category when the ad expires.
A feature, available in the near future, will allow users to inexpensively purchase a spot at the top of any category for a specific number of days. This section is different from the ads that are posted for free. Purchasing a spot in this section will guarantee that the ad remains at the top of the category for set number of days. This service should result in more visits and replies by users.