Market-makers make money off of the bid-ask spread, even if "trading fees" (i.e. commissions) are zero. Basically, the bid-ask spread is the difference between

Market-makers make money off of the bid-ask spread, even if "trading fees" (i.e. commissions) are zero. Basically, the bid-ask spread is the difference between the price the trader bought the stock at, and the price they sold it to you at.