Inventory Reductions interpreted as Sales
The Spy will continually check the inventory levels of a monitored product. The daily reductions are then displayed as sales.
Inventory level on day 1: 500
Inventory level on day 2: 470
==> Day 1 will be attributed with 30 sales.
Check Development of Inventory Levels to validate Numbers
By viewing the details page of a product (click the "binoculars" symbol on the right side) you can observe how the inventory levels evolved over time:
Accurate Sales when Inventory Levels are Reduced Steadily
In this example there are steadily 2-9 sales per day until June 6. These sales are, with a high certainty, fairly accurate.
Disregard Drastic Inventory Changes
The 45 sales on June 6 from the example above (high red bar on the right), however, might not be as trustworthy. In such cases you may want to "cross out" the value from your calculations.