The biggest problem with overselling is, you're so busy talking that you're not listening to your customer's needs. You can actually talk your way out of a sale by offering the customer too much information.
Think about buying a computer. Often computer salespeople spend so much time discussing RAM and megahertz that they never answer the prospective customer's true questions and subsequently lose the sale.
So what's the answer? It's easy--shut up and listen. Ask customers about their needs, and keep asking until you figure out how you can fill those needs.
Excerpted from Get Smart: 365 Tips To Boost Your Entrepreneurial IQ