Sure there are some bad sales people. And sure there are some bad customers. But I think in many cases there are customers that are not knowledgible enough about what they are buying. And when they buy it and find they made a mistake, they blame the salesperson. It's because they cannot blame themselves. People can't take responsibility for their own shortcomings. They always feel as the victim. Self doubt is just not easy. What is easy is to blame someone else. And claim they are the victim.
I have run across some questionable sales people. A lot of them are untrained, improperly trained, poorly trained (And Yes...there is a difference) or lack product knowledge. Some just make up facts they believe to be true. And some are just too lazy to research a correct answer. Not too hard to spot, if you are a prepared buyer. But if a buyer is not prepared, that buyer can expect problems.
It is often said that prostitution is the oldest profession. But first came marketing. Then came sales. Then prostitution. Without marketing and then sales, there could never have been prostitution.