you have a right to sell to a customer or not. if the customer has already had a bad record, e.g. bouncing checks, non-paying, etc, you can refuse to sell.

or if you want to sugar coat it a bit, taasan mo nalang ng presyo, at least di niya sasabihin na ayaw mo magbenta or di mo inalukan. ganon ginagawa ko eh :bwahaha: