I have done this ayear and a half ago or so, without using any Marketplace Modules, so I couldn’t tell you about thos. But basically I made a shoppingcart-module that had non-persistable entities that mirrored my product and that were being filled in with the required values of my non-persistable shopping cart attributes (amounts, total price etc). Then, only when the shopping cart gets payed for, I actually change the inventory numbers of the BackOffice persistable entities and change anything in the dB.
So the main idea of the seperate module with non-persistable entities is that you want no changes made to your dB as long as the shopping cart hasn’t been payted for and emptied again: this info should only exist in the session, which is why the non-persistables are perfect for this.
Best of luck!
Would also love to see some starter project with shopping cart functions.
Here are some options:
Searched github.com/mendix, nothing.
And my comment here: https://forum.mendix.com/link/space/other/questions/109337 a year ago.