But when you make a deal, deliver what you agreed on. When people like doing business with you, the word will get out that you are a good person to do business with, and you'll prosper.
For example, the guy who taught the accounting class I took used to be a used car salesman. I asked him how to tell if the car dealer was honorable or not. He said the ones in business more than 5 years were honorable, as they were getting repeat business. It takes five years to run out of suckers.
When I need some work done on my house, I'll ask the local real estate agent. They usually have a stable of contractors who'll get a house ready for sale, and they aren't using contractors that cheat their customers. When I ring them up, I make a point that Agent Bob recommended them to me, and so they know if they cheat me, I'll tell Agent Bob who isn't going to want his own reputation besmirched, and the cheating contractor will get cut off.