It has two interpretations. When there is a limited number of products offered (p. e.g. seats for a show) and sell more than the total stock or capacity, usually assuming that not all buyers are going to show up. Or when a buyer has already decided to acquire the product or service, but the seller keeps insisting as if he still has to convince him. It is formed by the prefix over- ("superior, surplus, above") sell ("offer a good for purchase").