How to keep track of the inventory available on different platforms?

Hi everyone,

a friend sells same item on few different platforms (Etsy, Depop, eBay, Vinted, Catawiki), and she would like to keep track of inventory (knowing where is something posted, and where not) and disable overselling (selling same thing on two different platforms).

So far she used Google Sheet, and she is managing it, but is there a tool or app that does that for her?

It can be both free or paid solution.

Thank you for the answers 🙂