In general there are two ways get an idea about the sales of competitor products:
1. Estimates based on the Salesrank
The sales can always be estimated based on the product's current salesrank. Due to Sellics' large historical database of sales tracking the tool can estimate quite closely which salesrank in a main category relates to how many sales. This is the method of the Product Detector and the Niche Analyzer.
As salesranks change on an hourly basis though, these estimates should be considered carefully.
2. Observation of Inventory Levels
In most cases it is relatively easy to track the stock levels of any product on a regular basis. The stock reductions can then be interpreted as sales - this is the method of the Spy, which usually yields in more accurate results.