Part number and model
The enquiry is always the same shape: a model, a part, and whether you have it. Keeping the part number as the item code and the make and model on the item means that is answered from the screen in seconds instead of from a catalogue. It also prevents the commonest loss in this trade, which is the wrong part sold and returned after it has been handled.
Three grades of the same part
Genuine, OEM and aftermarket versions of the same part sit at very different prices, and a shop entering them as one item can track neither stock nor margin. Each as its own item also protects you at the counter: the customer chose the cheaper grade and the bill records it, which matters when it fails and they come back.
Thousands of small items
With a catalogue this size, name search is slow and error-prone at a busy counter. Barcoding the fast movers — including your own labels on bins for loose items — makes the difference between a queue and a moving counter. The slower lines can still be found by number, and most shops end up using both on the same bill.
Mechanic running accounts
Your regular buyer is a mechanic taking parts for jobs he bills later. Small amounts, many visits, an account nobody totals until settlement. Because the ledger builds itself from the billing, none of it needs a second entry, and a weekly or monthly statement settles it before the figure becomes something to argue about.