Yesterday, a friend told me that Facebook allows users to host a shop as an app.
Is this allowed now?
Where can I find information about this?
Using Facebook Payment
Facebook owns a payment system available through the API to any developer wishing to earn money.
This payment system is only dedicated to games and app for which we get nothing but some virtual goods.
The Payment system doesn't allow you to sell and send anything.
Anyway, it would not have been a good idea... see:
Using a non-facebook payment system (Paypal, VISA...)
In that case, you are only displaying the content of your site under an app. An app which could be visible on your own domain (www.yourdomain.com) as well as on your Facebook app (www.facebook.com/yourapp). Your website/app could eventually lead to a payment page which is independent from Facebook. So, a webshop through an app is allowed, as soon as it doesn't use Facebook Payment! The only restriction from the Facebook policy is: