Confidential client information not to be given to 3rd party

I have a problem with auction nation on revealing information to 3rd party the is no confidentiality of information some of us are in business we buy cars and sell them but we end up fighting with our client because they give information to anyone phoning to request it .we buy cars and when our buyers find out where we buy them they phone auction nation to find out the price that we bought the car for and they give them causing fight between us.Can they please refrain from giving any information to a third party