In all honesty mate i'd tell him to fuck right off!

So what if he goes and tell his mates, they are probably all like him anyway and in reality you don't want customers like that!

The people that do use you with trust will be the ones you'll make money from and continue to grow a business with, at the end of the day they all know your a reputable breaker!

Just get your invoices marked as has been said and advise people your item's are 'sold as seen' but tested first