Ask HN: Help me build a simple bidding service for my university cafeteria

My university has made it compulsory to buy coupons for breakfast, lunch and dinner for 20 days every month. So a lot of students who buy them have spare coupons left. Others dont buy any, and end up borrowing, buying from each other. I would like to build a bidding system, so people can buy and sell coupons there. I have code for login, and am restricting to my university domain. How can I keep an open market, and let the bidders choose the current market price?

Thanks for any thoughts, suggestions or comments.