My father told me all the time growing up not to oversell. Hopefully the story he told me about it explains what he it means to oversell.
My father's boss was selling a man an edger, a piece of equipment used to neaten up grass near the edge of a driveway or garden plot. My father's boss had showed the man at least three times how easy it was to operate and make a nice, neat line in the grass next to the man's flower garden, and the man definitely wanted to buy it. Just to make sure he had closed the sale, my father's boss ran the edger one more time and accidentally ran over the man's prized tulips. Needless to say, no sale was made that day.
In my business, a dispatcher needs to close a deal as quickly as he can. If a driver agrees to haul a load for a given price, it is time to book it. It is important to be honest about what a load involves - does the driver have to unload the truck himself, how many drops will he make, how much will he be paid for the load - but not every detail needs to be discussed before asking, "So, do you want the load?"
As soon as someone agrees to buy a product or use a service, it is often a good time to quit "selling" and start moving toward closing the deal. In the case of loading a truck, it would be time to send the driver to the first pickup, or at least to give him directions. In the case of selling a product, it is time to head to the register or to get the credit card information. I cannot count the times I was ready to buy something in a store when the salesperson said, "Oh, and you will especially appreciate..." and they told me something that turned me off to the purchase. It is fine to help a customer or client realize why he would want a product or service, but once he expresses a clear desire to purchase, stop selling, lest you run over his flowers.