Trust in long-term business relationships is very important. Without it, less important deals will be made. Think of it this way: This random guy named Larry says that he'll buy a famous painting from you for 5 billion. He just needs to give you the money after he takes the painting home to see if it's real. You say no. There’s a chance Larry will escape in his private jet with the painting, cackling like a supervillain because he didn't pay a cent for the painting.
You will much more likely sell the painting to the honest guy named Steve who you trust, even if Steve'll pay less than Larry.
Financial problems aren't the only problems that could occur doing business with someone untrustworthy. Your reputation could also be harmed. Let's say that everyone suddenly found out that Steve is a criminal in Switzerland known for bank heists, which is how he got the money to pay for your painting.
Now everyone's going to remember that you did business with Steve, which will make your reputation come crashing down on you, with nobody wanting to buy your products and the CIA keeping an eye on you.
You have to be very careful with who you do business with. If you are, the reward is a profitable business and successful life.
Is asking the other person's opinion when selling something always the best idea? It will take up a lot of time you could be using selling products to other customers.