There's a difference between a sales talk and too much talk.
Know When to Stop Selling
Mark Twain wrote a piece about going into a revival tent and being so impressed with the preaching, that he determined to put 10.00 in the collection plate. After half an hour, he lowered it to 5.00. A half hour later, he decided he would leave nothing. When the collection plate finally came by, he took out 5.00.
In sales, as in preaching, sometimes the key is knowing when the sale has been made. It's much easier to talk yourself out of a sale than into one. Nerves, excitement, or desperation can often lead to saying more than necessary.
Something to keep in mind that might help, is that once you are in front of a prospect willing to listen, the sale is already made. Your only job at that point is to show how your product or service fills the need he's already acknowledged by letting you in the door.
One of the best ways to stop talking is listen. Ask questions. Sure the Whizmo Gizmo has 75 uses and you know every single one of them. What you also need to know is how many of those uses does your customer really care about?
Thorough knowledge of your product or service is essential for you. It is not for your customer. Your customer already knows his needs or problems, ask him about them. Once you know, then you can emphasize the features of the Gizmo that are the solution he is looking for.
Don't be afraid that if you don't point out every one of those 75 uses, that you will lose the sale. Holding back can often be effective, show the solutions for the main problems and keep a little something back.
Ask questions, present the solutions, ask for the order and shut up. If the client has more issues, you will have more solutions to offer. If your product cannot solve the problem, then thank the customer for his time and then find someone you can help.
Remember, time is valuable to you as well. If your product does not fill the customers need, then all the talking in the world will not change that. If you have the misfortune of convincing someone to buy a product that does not do what they need, the bad word of mouth advertising will more than offset any profit you might get from one sale. Sometimes the time to stop selling is before the sale is made.