Retail companies can sell items to private customers through retail stores. To setup a retail store, the retail company must own a piece of land and construct a retail store on it. A retail store construction can be built by a construction company. Once you own a piece of land, view that land's page to see a form that lets you create a contract with a construction company. If the construction company accepts your offer, they will immediately build the construction on your land for the price you have defined. Any retail store company may own an unlimited amount of such pieces of land.
To enable sales at a retail location, you must define sell prices on the company's management page, under the "Manage Retail" tab. Click on "Add Products" and select the item you want to sell, at what location, how many items to sell (or 0 for unlimited) and for what each price.