Selling game items by players

It could be a noob question… is it possible to sell game items by players by anyway? And if its possible, is there any standard way?
Thanks.

Do you mean like creating an economy system within your app?
I’m unsure if users would be able to sell items in exchange for real money (but I may be wrong) within your app, but you could create a fictional digital currency in your economy system, and that digital currency only belongs in your app universe.

I suppose you could create a secured website paired with the app in order to exchange the digital currency for real money.