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.
Possible reasons for inaccurate sales are mentioned in this article. In the above example Point 4 or 5 might have been responsible.