Well imastawka highly respected sellers on these boards leave the IDs of buyers in their follow ups so other sellers can benefit from their heads up and add to their BBLs on the chance they may buy from them one day.

Works for me, as I simply do not want to deal with the types of buyers that the follow ups are most usually  left for. 

From this type of information left by sellers trying to look after other sellers, I have added dozens of buyer IDs to my BBL that leave serial negative feedback / have multiple missing parcels / extort for partial refunds on a regular basis etc etc