Businesses used to have to spend thousands of dollars on expensive cash register systems connected to their computer systems in order to collect cash and adjust inventory at the time of sale. These are called point of sale programs.
Just to give you a little background — in case you're not familiar with how point of sale programs and inventory management coexist — there are two ways to track inventory: One is the periodic method, and the second is the perpetual method.
The periodic method requires you to periodically close down the store and count inventory. You've probably gone to a store that actually closes for a certain number of hours or even a day or more to count inventory. This is often done on a monthly basis. Some retail stores count daily or weekly. It all depends on the volume of the business.
Companies using the perpetual method actually adjust inventory count each time a sale is made. They do this using computer software. Even if a company does use the perpetual method, it is a good idea to periodically check the true inventory count because of spoilage, breakage, or theft.
Today, thanks to mobile devices and apps, a company can download a point of sale program to any mobile device and have the same controls for tracking sales and managing inventory. Many apps also include customer relationship management as well, to make marketing and promotions targeted to a customer's buying habits even easier.
You can find an excellent review of ten systems at Software Advice. You do want to be certain that your point of sale program is compatible with your accounting system, so you don't end up with a major project inputting all the data from your point of sale program into your accounting system.