Exactly, you are handling the sale and you want to make money with it. There is responsibility involved. You are the provider. You can’t just say: you guys figure it out if something goes wrong (chargebacks, lost deliveries, illegal activities and so on.)
Not to speak of the technical implementation. I can’t even begin to imagine how something like shipping would be handled, if any community user can ship any kind of product and any number of products from any point on the globe to every other country. There are of course many web stores that can pull this off, but they do nothing else and might invest millions in venture capital to develop all this. There is so much more involved than just to add a button for users to sell products. Technically and legally.