Stock control system

It’s actually not that easy a question to answer.
What you choose, will actually depend on where you sell and exactly what you want it to do.
There are literally loads of different platforms out there, but I don’t think that any of them are particularly cheap as such.
Take a look at the below link, it gives you possibly the most well known ones.

https://www.brightpearl.com/best-amazon-inventory-management-software