I read this thread yesterday, and thought about it today while I was doing yard work. I am an hvac tech (for the last 20 yrs) and also i talked to a coworker that was on call today, about this situation.
the following are some ramblings and thoughts about our trade and its comparison to the auto industry that myself and my coworker came up with.
first of all, most people (household or family) have one heating appliance and one cooling appliance. but they have 3 or 4 cars.... so volume is huge here. also we guessed that on average a furnace or air conditioner will break every 3 to 4 yrs. where cars have more moving parts and will break more often, especially older cars. plus its cool to rebuild older cars.... hvac equipment just doesn't look that cool with a new paint job and 22" wheels....
I live in a town of 22000 people. in my town there are 2 hvac supply houses (each repping their own brand of equipment) two plumbing supply houses and one refrigeration supply house that also reps some hvac stuff.... these are the only supply houses in a 50 mile radius.... and remember each supply house will have some univerasal parts, then mostly stock oem parts for the brand they carry......in the same radius are at least 10 new car dealers... in my town alone are 5 auto parts stores.. plus remember each dealer will sell parts over the counter. also note that the 5 auto parts stores specialize in carrying common parts for every make.
my point here is this is a supply and demand thing... there is no "deal" that has been made behind the scenes that keeps homeowners from buying parts over the counter..... the demand is just not there.... we live in a free country... if it was profitable somebody would have done it... as has been stated the supply houses I deal with are not setup to charge tax...
remember, trying to buy parts from a contractor is the equivilant to trying to buy them from the local independent auto mechanic. more than likely he is already busy selling parts and labor, stopping to sell "just parts" over the counter is a pay cut.. plus most good mechanics/techs are busier than they want to be..... selling parts over the counter is just less profitable, this may sound bad, but I promise you they are in business to make money....
the company I currently work for will not sell parts over the counter, my bosses stance is that there is too much liability (meaning the customer could hurt themselves) also in my state we are required to have a epa 608 license and a mechanical license.... the mechanical license is our lively hood and could be lost just like a doctor could loose his license. technically if a customer hurt themselves trying to install a part we sold them they could file a complaint with the state and we could loose our license. I'm sure it sounds funny, but it is real!!
the company I used to work for would sell any parts including equipment over the counter.... it was all $$$$ to them.
lastly I don't really have a stance on the subject...I get it, I do lots of work around my house, in fact I'm getting ready to have a spray foam guy insulate the basement of my house and the only reason I am having it done is because I don't have a spray foam machine



in the 13 years ive lived here it is the only thing ive hired out for except roofing. ive added onto my garage, built sheds, patios, porches, redone bathrooms.... you name it ive done it.
I guess the bottom line is in most areas I don't think there is enough volume in hvac to have a dedicated "auto zone" or "advance auto parts" but there is plenty of room on the internet for places like "amazon" and "supply house.com".....