If I understand your question correctly, you want to know if you can buy a for sale by owner home that is listed here. First of all, homes listed for sale on any website have to be purchased from the owner, who may or may not be using a real estate agent to assist them in the sale. These websites, as well as the multiple listing service and other websites, are just avenues for advertising the home. Second, Trulia only posts listings from Real Estate Agents, it is not a 'for sale by owner' site. Also, there are some listings here that are fed from Reaty Trac, which posts pre-foreclosure listings. Those homes may not even be available for purchase at all, it is merely a listing of homes where the owners have not been able to pay their mortgage and the home has gone into some stage of the foreclosure process.
The best thing you can do if you want to purchase (or sell) a home is to contact a good, local real estate agent to assist you in the process.
I hope that helps.