I have started an item shop in my classroom. Students can use GP to buy one-time passes that function similar to other class powers. Although I love the fact that each class has a different "skill set," I wanted to allow each student to have the possibility to experience some of the other "powers."
This shop can easily be adapted to fit whatever you want to "sell." I printed out little tickets and laminated them so that students will actually get a physical item when they buy. They hand in the ticket when they want to use the power.
I teach middle school, so my shop is open at the very beginning and very end of class (or between classes) so that we are not interrupting instruction to deal with the item shop.
Also, this post could function as a suggestion to add an in-game item shop to Classcraft so that the gold and tickets/powers could be managed automatically.