I couldn't agree with you more, but what you don't realize is nine times out of ten, the customer a parts salesman is talking with really is an idiot.
Let me explain what I mean by idiot. The customer walks in the door, and asks for the 'biggest camshaft he can run and still use a stock torque converter'. Or he walks in and wants a set of headers for his stock 350 Camaro, with 2" primary tubes. Or he wants a long-runner, single-plane manifold and a 750 double-pumper for the same engine. (Yes, once again I managed to work in three hyphenated phrases into one sentence!!

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At that point, what is my job? To remember the customer is always right and sell him parts that will turn his car into an absolute pig? Or to get a sense for what he is really trying to do, and what his budget is like, in order to sell him the parts he really needs? Seriously, we had a title for those kinds of customers. Since they had become 'experts' by reading the catalogs from the Ohio warehouses, we referred to them as 'wish-book racers'.
I wanted my customers to be happy, to give us good referrals, and to give us future business, so I always opted for Door Number Two. Never mind that I may have as much as $8,000 - $10,000 worth of valve spring orders waiting to be shipped, with UPS pick-up just 30 minutes away. Never mind that I might be on the third invoice of what would be a eight-invoice, $15,000 race motor. Never mind that the phone might be ringing off the hook, with even more valve spring orders, as well as the occasional caller wanting to price the 'biggest camshaft he could run and still use a stock torque converter'. I would stop whatever was happening and start trying to help this customer, who is standing in front of me.
Question 1 - OK, what is it you are trying to build? Is this going to be something you are going to drive every day, something you are going to drive on weekends, or...?
Of course, they were going to drive it every day, but they would always want the rough idle and they also wanted to use 87 octane gasoline.
Question 2 - Have you made any other changes to this engine?
They never knew what had been done, because they had just bought the car, and the seller told them we had charged him umpty-thousand dollars to build the engine.
Question 3 - How much compression does this engine have?
No clue.
Question 4 - How much rear gear does the car have?
No clue.
Now hear me out. I am not saying or suggesting that anyone here could be so thick, I am just telling you that nine times out of ten, this was the mentality of the customers that would walk in our door. Those people had to come from somewhere, aye?
I would get valve spring customers call me, wanting to know what valve spring they needed to run. Talk about walking a high wire, without a net, that stuff was crazy.
Our catalog and our Web site had an illustration that showed what different terms meant. But no one ever bothered to look at that illustration and to comprehend it. The absolute bare minimum I needed to know would be cam/lifter design (hydraulic tappet, solid tappet, hydraulic roller tappet, solid roller tappet), diameter of the spring pocket, actual lift at the valve, and total installed height, with no shims in the pocket. Leave any one of those variables blank, and there is no way to make a proper recommendation.
It would make you nauseous to know how many people would call, every single day, and admit they were measuring dimensions with a wooden ruler. I had a guy pitch a shoe at me, because he had to cut his wooden yardstick, in order to get me a dimension. And no, I am not kidding.
We had a spring that we offered as a drop-in replacement for a stock small Chevy. The spring was great, up to .500" lift. Run that same spring at .510" lift and they would break, every time. So we carefully added a warning, to say that spring was useless beyond .500" lift. We tried to include as much information as we could with our springs, so we would always get the guys who were 'smarter than the average bear' (see where this is leading?) ordering that spring. They had looked at solid height on the spring and they could reverse the math to see they could run as much as .560" lift, so they would call and order that part number for their .550" lift combination. But since they were smarter than the average Joe, they would just call a dealer and order the spring by part number. And without fail, within 48 hours of installing the springs, they would be on the phone to me (not to the dealer, mind) pitching a hissy fit at me, because my 'junk-ass springs' had broken.
My procedure for those calls was always the same. Put the guy on speaker, and go on working on whatever I had been doing when the call came in. Say, "Uh-huh," and, "Yeah," every few seconds, and just wait for the guy to sputter to a stand-still. Then, I would pick up the phone and ask the key questions. What kind of cam/lifter design, what diameter spring pocket, how much lift at the valve, and how much installed height. Now mind, this was always the guy who was smarter than average, so those questions would wind him right up, making him think I was belittling his <cough> 'superior intelligence'. Put the phone back on speaker, until he wore himself out again and then ask, "Are you aware we specifically do not recommend using that spring with that much valve lift?" The put the phone back on speaker, as they would wear themselves out, explaining to me how 'smart' they were, and how they had done all the math, and how that spring would not coil bind until .560" lift, etc., etc., ad flipping nauseum.
Seriously, what do you say to a customer that is so smart they could not even read the warning on the product?
I've had championship-winning engine builders throw bitch fits at me, because they were not smart enough to calculate coil bind clearance.
And again, I am saying I am not trying to cast aspersions on anyone here, but I spent 37+ years in that industry and I dealt with enough customers to know that more times than not, the customer is definitely not right. The customer thinks he knows enough to make good parts selections, but if left to his own devices, he could mangle ball bearings in a sandbox, using a rubber hammer. I spent 25 years, working as the sales manager of that company. But when push came to shove, I would unload spring components off a truck in a rainstorm. I would do finishing work on spring components. I would assemble springs. I would stripe springs. I would pressure check springs. I would box springs. If you called to buy springs, I was the one you talked with. I was also the one who put your order in a box and shipped it to you. I helped customers diagnose spring problems at the race track. I helped customers change springs at the race track. I was the crew chief on a race-winning, record-holding car that ran those springs. If you attended a P.R.I. show, odds were you would be talking to me at the show. I knew those springs inside and out, so why wouldn't you want to take advantage of that knowledge? I sold springs to racers in Top Fuel, racers in Top Alcohol, racers in Comp, racers in Stock and Super Stock, to Indy car racers, to racers all over the world. Racers who not only won races, but also won championships.
It's good when people try to educate themselves on anything, but it's also good for them to sit back, relax, and allow the experts in a given field take care of them. It's like the people who jump on here and ask what stall torque converter they should run.

They have absolutely no idea how critical a decision that is, and think the selection is as simple as running whatever the next guy is running. Get on the telephone, call the manufacturers' tech lines, talk to people who build and sell converters every day of the week, and ask them what you should use. Personally, I think the majority of people should just run the stock torque converter, because they bought the biggest camshaft they could run with a stock converter.
Now, you may have a better idea of how things look from the other side of the parts counter.