pontig/lokalino_mlud — reverse-engineered prompt

Build me a simple web app for a used schoolbook market called MLUD. It should help replace the current paper based workflow with something staff can use on phones or desktops.

I need three kinds of users. People who bring books to sell, people who want to buy books, and operators who run the market. Sellers should be able to add books through an easy form, ideally by entering an ISBN and having the book details fill in automatically when possible. Operators should be able to see inventory, register sales, mark books as sold, and keep track of what is still available. There should also be a basic way to handle unsold books, returns, and payouts or notifications for the original seller.

Please make it as a web app with a React frontend and a PHP backend using MySQL, since that is what this project is meant to use. Keep the design clean and responsive, and make the flow simple for non technical volunteers. If anything is unclear, check current docs online and make sensible choices.