Yeah, I know what you mean. I was that counter-monkey for a while back in my wasted youth.
Once you know who the mechanics are, you know they know what they want. Then there's the guy who's looking for a plug to fit the little hole beneath the shaft of his Ford 302 water pump, you know, the one that's leaking coolant. The guy's dead certain that the plug fell out and he thinks you're scamming him when you try to explain that's a relief hole to let coolant escape when the pump seal wears out. Sold a heck of a lot of water pumps for Ford 289s and 302s because they hung 3 or 4 pulleys and the fan off that water pump!
Then there's the guy who wants to do something different. Like doing something with an antique engine or a diesel or a generator, or all three! After selling spark plugs and water pumps and transmission modulators and thermostats and radiator hoses and v-belts and brake shoes and mebbe a radiator, when something like that comes your way, you get to use your brain and whatever knowledge you've accumulated looking through parts catalogs.
Quinn