I think setting up a back office system and using service providers like ModiSoft, Skupos and others would be a good option but they charge a fee subscription fee of around $59 to $79 a month.
I Used them in the beginning, but I found it easier to use weekly inventory templates for each vendor to understand how many cases/cartons/pcs I'm selling and ordering each week/month.
But looking forward to see what others are doing/using.