Its better to work with real currency payments through an website than a desktop application, so you must search a little of webmaster knowledge for a start. Afte that just take a look for Paypal API usage tutorials.
I recommend you to take use of Intersect CMS for a website base too.
When the transaction is done, just send API requests for your server to give the shop items, everything you need is on the docs https://docs.freemmorpgmaker.com/en-US/api/v1/endpoints/players.html
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