Selling – Simply click on the Sell Something button and follow instructions to list your item. The minimum you need is a location, price and picture.
Buying – Simply search what you are looking for. By default, Facebook will display the last items you searched for. If you see something you like, click on the image and you will be taken to more details.
Facebook gives you a page similar to the above. You can click on the Is this available? to open a new Messenger chat window or click on Message at the bottom. It is up to you to decided and ask questions about the item. You are also responsible for the collection / payment of the item; Facebook is just the equivalent to an online advert.