If you are here, then you've tried uploading a file with the UPC codes formatted in scientific notation. When this happens, Wholesale Inspector cannot properly determine which Amazon listing the product belongs too.
If you are working with Excel, the best way to remedy this is to:
Open the original supplier file in Excel
Right click the column letter that contains the UPC codes
Under the Number tab, click Custom
Enter "000000000000" in the Type box
Save as a CSV then rerun through Wholesale Inspector
If you are in Numbers, the process will be quite similar:
Open the original supplier file in Numbers
Left click the column that contains the UPC codes to select the column
In the Format sidebar, click the Cell tab.
Under Data Format select Text
Click File -> Save As
Choose CSV as a file type
Rerun through Wholesale Inspector
Why does this happen?
The root of this is that Excel does not handle UPC codes well. It tries to be "smart" and convert all UPC codes to scientific notation. When it does this, it zeroes out a large number of the digits in the UPC code, rendering it invalid. So for example, if 123399180666 gets saved in scientific notation, it may end up getting saved as 123390000000.