The answer is simple:  If you contact the Seller's Agent regarding real estate for sale they must act in the best interest of their Client.  If you are not also their Client who has the advantage?  

A.  You, as a Buyer

B.  Seller  

Even if you have interest in real estate that is a For Sale By Owner (FSBO) a Realtor can save you from major problems if you want to make a purchase.