For many here the rules are second nature. Just as we as pilots know about weather minimums and altitude limitations. It is just part of the daily grind.
We don't get to make the rules but we are in a regulated environment and are expected to have a knowledge of them at least.
It is about competency as you suggested.
As for buying the items and where to get them from, well yes this is exactly about that.
The area that owners seem to have least knowledge about is the requirements for the supply and fitting of aviation products.
That is the very question that was asked here.
You mentioned 'simply'.
Part 21 has to be one of the most difficult regulations to read and understand. I would have thought it is anything but simple.
I do see many owner produced items. I have already mentioned that many do not end up getting fitted to a certified machine.
Just because the owner has spent money buying something does not in any way negate the requirements. It is a bit much expecting the LAME to sign off on an item that is 'wrong' for possibly any one of a number of reasons.
I do get to see it far too often.
It is much easier to ask the LAME for some advice before dragging the credit card out.
Some may seem to think that the Part 43 and Part 21 regs don't weigh as much as Part 91. It does to the guy that has to put his licence on the line every day. The thing is it so easy to operate within the rules.
Just like stopping at a stop sign for example, what is to be gained by driving through it ?
The point being that you may only stop if you are aware you have to. Some may not even see it !!
A lackadaisical attitude may see pilots flying about without medicals and renewals. Most however would probably bother to meet those requirements.
Same rules...
The original question was about where to buy some aircraft tyres.
It took a long time before the thread had any mention of the requirements and possibly substandard and bogus parts.
Someone did mention Aviall may not like selling directly to the owner... perhaps there is a reason for that.
I am not suggesting that everything from Spruce is suspect. You cannot just blindly go out and buy an aviation product and then proceed to fit it to a certified machine though. Life was never that simple.
The onus is on the individual certifying.
He is accepting all responsibility for the Part 21 and Part 43 requirements.
The problem we often get is that it is the 'hangar gremlins' that mysteriously 'fix' the worn tyre.
I am not sure about the competency of those blighters, in 40 years I have yet to meet one. They don't even sign their name so they really are elusive characters.