when a contract between a customer and a retailer and the contract is signed by both parties,it states on the contract that all personnel information is private and confidential.

if the buyer defaults on payment,does the retailer have the right to pass-on the buyer's personnel and confidential details to the debt collector or is the seller in breach of the contract?

Once you have entered a credit agreement it will be stated in the contract T&C's fine print that in the event you default, you agree that your details will be used or passed onto the appropriate credit reference agencies or other relevant debt enforcement agencies.....so the answer to your Q is Yes, they do have this right.