Buyers attempting off-platform deals or other manipulative scams.
Sellers typically add a buyer to this list for one or more of the following reasons: blocked buyer list
Since you did not specify which platform you are using (eBay, Amazon, Etsy, etc.), here is the most common answer based on how major e-commerce platforms handle blocked lists and reviews (specifically eBay, where this is a frequent question). blocked buyer list
Competitors who purchase items solely to leave negative feedback or tie up inventory. How to Manage Your Blocked List (Platform Guide) blocked buyer list