Hi,

You can create your modules and sell it in the Odoo store, but it is not like a subscription, it is a one-time payment method, you can create a module and just sell it for an XX amount.

If you need something like a subscription, you can set up your own saas instance and sell it is a service in it.

Also, You can check the In-App Purchases: https://www.odoo.com/documentation/11.0/webservices/iap.html

As you are planning to sell the module,  better to create from the code rather than doing it from studio and export.

For creating your own custom module, you can refer to this blog: How to Create a Module in Odoo 12

Thanks