This is a good question. I can see where you confusion is. While there is some overlap between sales and marketing, there are two important differences.
First, marketing takes place way before sales. To be able to sell a product or service, there is a lot other work that goes on. For example, you will need to price things in an appropriate way. You will need to do research on what products or services people want and how much they would be willing to pay. Also you would need to know things like the competition, so that a company can actually sell something and make a profit.
Second, marketing does not end with selling a product. Marketing does not merely want to make a sale, it wants to create loyal customers.
Well a logical answer may be,
"To sell something, you must
have a customer first. Selling begins with
a product or a service, whereas
marketing begins with the customer."
Hope it helps