While Zillow For Sale By Owner is free to list, the user interface on your Zillow property listing page is designed for interested buyers to click 'Contact Agent' in order to inquire about the house and schedule a showing. The user experience is designed to make it difficult for the buyer to contact the seller directly. Zillow does this because agents pay them for for buyer leads.
This places the seller in a situation where they either need to pay the buyer's agent commission, or they need to have the buyer pay the buyer's agent. Both scenarios place the buyer in an uncomfortable situation since they were the one to find the listing and the seller may not have priced the listing to include paying the buyer's agent. If the seller is going to pay the buyer's agent, they might raise their price. From our experience, this complicates an otherwise straightforward transaction flow.