No they wouldn't. They would be breaking the law if they refused to sell to someone because they were black. If they refuse to sell to a person because that person is not a member of a particular trade they are not breaking the law.
Taking another example. The local cash and carry wholesaler will sell paracetamol and ibuprofen tablets in bulk to my wife. They won't sell them to a neighbour who is black. The cash and carry wholesaler is not breaking the law.