The number to watch
Wire stock in metres against coils purchased. Wire is bought in coils and sold cut, and offcuts are where the figure quietly drifts. Recording the sale in metres against a stock held in the same unit keeps it honest, and the gap that shows up is real loss rather than a rounding error. In a shop selling several sizes and brands, that gap across a year is usually a larger number than the owner expects.
Wire by the metre, coil by the bundle
Electrical wire is bought in coils and sold by the metre, and cut lengths leave offcuts that never appear in stock. Holding the conversion on the item keeps the figure honest, and recording the sale in metres is what the customer expects. Cable size, core count and brand are all part of the item, because a 1.5 and a 2.5 are different products even when a customer asks for both by the same word.
Thousands of small fittings
Switches, sockets, MCBs, holders, boxes and fittings run to thousands of lines in a dozen brands, most of them small and similar. Barcoding your fast movers, including printing your own labels for the bin, makes the counter considerably quicker than searching by name. It also makes the stock figure survivable, because nobody is going to count that manually.
Electrician and contractor credit
Your regular buyer is an electrician drawing material through a wiring job that will be paid at the end. Many small visits over several weeks, which is exactly the pattern that grows unnoticed. The ledger builds itself from the billing so nothing is entered twice, a credit limit stops a stalled job continuing to draw, and a monthly statement settles the account while the job is still fresh.