Sidney,
I for one can tell you that we/I have had mostly excellent experiences with salesmen. Yes a few were far from up to snuff as they had little or no education or real knowledge of what they were selling. Of course all 3 were in automotive or RV sales as expected. It was so obvious that they had very little or zero experience using what they were trying to sell and looked worse than stupid. It's something that's hard to forget and is repeated to others often when the subject comes up.
Knowing the product etc you are selling and being totally honest is paramount to being a good desired salesman. Know the actual construction and be able to display it directly to the customer. Know the positives and the negatives and do NOT deny or gloss-over a presented documented product's shortcoming facts that are expressed or written. Really makes a huge difference in earning trust!
Being honest and admitting that you do not know something but will find the answer and get back ASAP and do what you say is next.
Don't over brag or exaggerate the product or service you are selling is also very important as a halfway intelligent customer can see thru the smokescreen and will be lost for ever and tell others very often.
If your product or service is actually good, it should sell very well if competitive and not overpriced or over bragged. Notice that this has played out very well in the automotive industry and nearly as well in the RV segments. Some products lag behind in sales for decades and there's very good reasons and the buying public is not stupid as they know! The best products virtually sell themselves very well year after year and some for many decades and require little salesman interaction other than to answer customer's questions honestly and help them with the sale proceedures.
One of our bad experiences was also very shady and got busted in a court trial and served time well deserved. 100 gorgeous apples in a bushel is easily infected and ruined by that one rotten apple and it doesn't take long and/or won't be forgotten! Remove the rotten apple imediately or the entire bushel is tainted or doomed.