FanBox members can sell and buy items on the internal marketplace. You can list items and advertise them, either at a fixed price or as an auction item. Members can buy with funds they already earned on the site or with funds from their PayPal and credit card accounts. No one gets the payments d=credited to them until the buyer has confirmed they have received the goods or services, so the security is good for both sides.
Best starting point is to view their guides on the topic - there is a large number of guides in the Learning Centre.
Of course you do have to get an invite to join these days - that system was introduced to make it harder for the scammers and other pests to get an account, but if you know someone on FanBox ask them to give you an activation code to use when signing up.