Validate if values associated with the same part number are in descending order

I think using Conditional Formatting would be your best option here.

For this example, I'm going to assume this is a list that starts in cell A1 and runs down column A. Create a new conditional rule for the range of cells, and type the following into the rule:

"=A2>A3"

Then change the format to whatever kind of text formatting you want to highlight the error.

Edit: Adding attachment as example.
CondFormat.xlsx