A spreadsheet is a fine start. This is where it usually stops working.
One truth, not four files
Stock in one sheet, sales in another, cheques in a notebook. An ERP posts one sale to stock, receivables and the ledger together, so they cannot disagree.
Rules, not memory
Tax, discount limits, credit limits and gapless invoice numbers are rules in the system. In a spreadsheet they are habits.
Who can see what
A spreadsheet is all or nothing. An ERP lets the cashier sell without seeing cost, and the owner see everything.
When a spreadsheet is still right
One person, a few dozen items, no staff and no credit. Past that, the time spent reconciling is the cost.