If you are a for sale by owner the post is not allowed, you can however try blogging about the property; if and when you do list with an agent, he/she can post it for you, ask; or consider any flat fee realty company that feeds into the site.
As the agent said below, Trulia is a feeder site. To my knowledge they do not take listings directly. They pull their information from the MLS. There are a few ways to get into the MLS. Either through one of the many full service brokers available, a discount broker, and a for sale by owner site. You can also pay to be directly put in the MLS.