I had the same experience in Arizona. HVAC supply shop was 150yards down the road from my house. Had the AC fail to turn on in the middle of summer. Went up on the roof and troubleshot it with my multimeter and found the fan start capacitor was toast in about 3 minutes. Pulled it off and walked over to the supply place and had to argue with the guy for 15 minutes before he would sell me the $10 part that was right there on his shelf. He didn't want to sell me anything because I didn't have a license, but I told him I'm fixing my own AC and don't need a license to work on my own house. He finally relented when I told him I live right across the street and you can watch me put it in if it makes him feel better.
Yes, there are people that ignorant/stupid.
In the past, I had to call up the local codes (Revised Code of Washington [RCW'S]) that, specifically, state we have the right to do our own work and do not have to seek permission from our agents to do our own electrical work, HVAC work, plumbing repairs. . . . and provide it to the business.
Here, putting that $40.00 switching unit on would cost me around $200.00 in labor, or I could buy the part, photo the color coded connections, kill ALL the power (even with the external circuit opened, the unit would still get 24VDC from the handler, so it's just smart to kill the house breakers, the HVAC breakers, AND check for voltage before the swap.
That the code had to state, in plain terms, we had rights, says how ignorant much of the public is, including so called experts.