After 25 years in sales, the first & foremost lesson is this:
Respect your customers. All of them. Even the ones who don't buy anything major from you. If you don't practice common courtesy at all times, your attitude will grow and fester. Finally, you'll sit in an empty shop wondering where everybody went. I once solved a $15.00 problem for a guy. It turned into a $75,000 sale a few months later. I walked out of a local bike shop in 1979 due to rudeness on the part of the owner. I've never been back. Bought a few bikes & a lot of parts over the years. He never even had a chance at my business.