State-of-the-art people counting software such as VisiCount compares traffic data with conversion data to calculate the conversion rate. You can import your sales data from the source of your choice and allow the software to perform the calculations.
Since you get to determine the conversion you want to track, you can use people counting to measure a wide range of results. For example, if a library uses people counting, the staff can calculate a circulation rate by comparing traffic to the number of people who check out books.