To sell is to exchange a thing or a service for money. You could sell lemonade from a table on the sidewalk, or sell hotdogs at a baseball game.
When a business or store carries certain products for purchase, they sell those things; a car lot sells automobiles, and a bookstore sells books and magazines. Sometimes people say that an item "sells at" or "sells for" a certain price: "Normally, those candy bars sell for a dollar each." When something sells out, it's totally gone, but when a person sells out, they give up their principles for money or popularity.