The line that sells twice a year is why they come. An obsolete cartridge sitting for eighteen months looks like dead stock in a report and is the reason a heating engineer drives past two competitors, so those lines need judging differently from fast movers.
What fits what is usually one person's memory. When the counter manager retires, so does the knowledge, and writing compatibility notes against products is what keeps a shop's real advantage inside the business.
Ordered-in parts belong to a stopped job. Somebody is waiting with a bathroom apart, so a special order needs a promised date, a chase and a phone call the moment it lands.
Half of what goes out comes back. Plumbers buy three fittings to be certain and return two on Friday, so a returns process that is quick and unresentful is part of the service rather than an exception to it.