If homes are for sale by owner, usually agents don't get involved because there isn't a commission to be collected. For Sale by Owner generally means that the owner is going to take care of everything themselves, instead of going through an agency. Unless the owner was willing to pay an agent a fee for finding a buyer for them, then there would really be no point for an agent to get involved.
If you're asking if you can advertise on Trulia as a For Sale by Owner, the answer is no. They don't allow FSBO listings on this site as it is meant to be a place for agents to advertise, find clients and connect.