You can post free ads online using the listing website Craigslist.com. Additionally, you can post your ads at no cost on websites such as Backpage.com and Recycler.com.
Craigslist allows users from anywhere in the world to post an ad for free; however, Craigslist limits postings to only one city within 48 hours of your last posting. Backpage is similar to Craigslist except that it allows for multiple listings in several locations at the same time. You can also update your listing within the same day. Recycler allows you to post on its site and offers a Twitter and Facebook integration feature.