You cannot actually buy and sell items on the Facebook Marketplace. You can just find the products (or list them) and get in touch with the owner to arrange payment or negotiate the price.
No payment processor supported. No shopping cart, no tax calculation, no nothing that you expect to see as a seller.
All you have to do is to always ask the buyer to pay before service. Those that trust you will always go as a far as doing business with you while the doubting Thomases will say no. Facebook has a big number of people and selling on it can be a great experience.