The Spy will generally provide a very good idea of a product's actual sales.
In some cases, however, the Spy's method will deliver less accurate results:
Inventory Level above 999 Units
For these listings the tool is not able to register any inventory reductions and thus sales. These cases will display a yellow exlamation mark.
Limit on the Amount of Units that can be Ordered per Order
If a seller has set this kind of limit, the Spy won't be able to register the current inventory level and thus cannot track any reductions above that set limit.
Simultaneous Increase of the Inventory Level
On such days the tool is not able to register all inventory reductions and thus sales.
Manual Reduction of the Inventory Level by the Seller
This kind of reduction will be interpreted as sales, as they cannot be distinguished from normal reductions.
Damaged goods in Amazon's warehouses
These products will also cause a product's inventory levels to decrease, which the Spy will then interpret as sales.
These orders will temporarily reduce the amount of units available on Amazon, even though they do not lead to sales later on.
Also, please be aware that Sellics can only offer sales numbers for the periods in which the product was being monitored.
Tip for 1 & 2: Monitor Similar Products or Use Niche Analyzer
Points 1 & 2 usually do not make any kind of sales tracking possible. In such cases you may want to replace it with another similar product, which should also be ranking on the first page of the search results in that specific niche and ideally have a similar salesrank.
Alternatively you can enter the ASIN into the Niche Analyzer to obtain a sales estimate based on the current salesrank.