A few things.
1. I'm not trying to upset you. I'm simply stating my opinion. I'm not out to get anyone here...
2. You say I am intent on reducing your pay. I said, in my second post, "I have had people tell me they will charge a little more and I tell them that is fine, I understand." Again, I'm willing to pay a little extra labor if that is what is required to do business. In Tracy's post he says he charges his clients to source parts for them. That is extra labor that he makes money on. It likely makes up for money he does not make on selling parts (Tracy?). More generally, I think it is fair to say all industries are under considerable pressure from consumers and competitors alike. I do not think any "conventions" are safe and most will continue to change. I think we all benefit and we all get hurt by this change one way or another. I am in Real Estate. Although the perception is that the commission on a home sale are huge. I think most would be surprised to learn that the average brokerage nets only about $500 profit per home sale (yep, that's all). A real estate company has to sell a lot of houses to make a living. So, if you are feeling that pressure - you're not alone.
3. You talk about convention. Obviously, it is your convention you speak of. We all have a unique perspective on what convention is. Since myself and others have supplied our own part and have never gotten any push back, others must have a different convention than yours. It could vary by shop type, region or many other variables. Who knows.
4. You say "real business". Again, your definition. I'm sure the people that have done work with customers who have supplied parts for people like myself, Tracy and others consider their business real. In Tracy's case, I'm sure the MB dealer consider themselves to be a "real business".
5. Truthfully, I have never even considered the warranty question. I guess it is because I have never had an issue with it. However, in the past when I did have a mechanic install a part which later failed they have always charged for the labor to replace it but replaced the part under warranty. You obviouolsy go the extra mile for your customers and I'm sure they appreciate it. I'd use you - as long as you didn't over charge me for parts.