The whole point of a business is to buy and sell, whether it be entertainment, make-overs, food - anything. Sims are more likely to want to buy things that have lower prices and lower markups, so it helps to have at least one controllable Sim available to direct them to more expensive items. They will not do this autonomously, it must be directed by the player. Also, customers may come in not knowing what they want, in which case a controllable Sim can use the "May I Help You?" interaction. An employee assigned to sales may do this autonomously, but probably won't, so players need to watch for customers who need to be helped.
The more expensive an item or service is, and the more highly it's marked up, the harder it will be to sell. Still, regardless of price and markup, making a sale will generally require more than one sales interaction.
When a Sim is considering whether or not to buy something, a green bar topped with a simoleon symbol will appear above his or her head. When the bar reaches the top, the sale is complete, and the Sim will buy the item or service.