Revised:
Where spreadsheets work
A spreadsheet is fine for a few hundred stable products, one person editing and one sales channel. It is free, flexible and familiar. The problems start when several people change stock at the same time, when products arrive faster than they can be typed in, or when stock must match a webshop in real time.
Warning signs
You oversell because the webshop and the warehouse disagree. Products sit in the warehouse because nobody had time to list them. Different people keep their own copies. You cannot tell where in the warehouse an item is. Partners ask for an up-to-date stock list and you send a file that is already outdated.
What an app should add
Beyond counting, look for phone-based intake from photos and barcodes, automatic listing drafts, bin locations and labels, two-way sync with your webshop, user roles and a history of changes. For wholesale, a live partner catalogue replaces emailed price lists.
Switching without chaos
Start with new arrivals: list them in the app while the old stock stays in the spreadsheet, then import the rest in batches and count one area at a time. A free trial is enough to test the workflow with real products before committing.