Trulia does not have agents, it's just another avenue for agents to advertise and 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.
All agents on Trulia are not agents of Trulia. We all work for different real estate companies. My advice would be to look for an agent in the local area and "shop" around for one that'll do the most for you. :)