As a CPA, I am bound by professional ethics to protect the confidentiality of any information provided to me by a client. Within certain legal limits, tax planning information provided to me by clients is protected by accountant-client privilege similar to the attorney-client privilege. However, any data relating to tax preparation or other (non tax planning) services could be obtained by the issuance of a subpoena by a court of law. In my 30+ years of practice, this has never happened. However, there are substantial limitations on the accountant-client privilege of confidentiality and I encourage you to review my comments on that subject.
I do not sell or rent the use of any client information at all - period.
I am not affiliated with any other companies or persons with whom I would be required to share any information about my clients but I may choose to seek help from members of the International Tax Compliance Group on those occasions when their help would be beneficial to the client.
I work entirely by myself at a home office and there are no employees who would have access to any client information.
I do retain client correspondence and tax returns on my computer but use a firewall system (and other means) to protect my computer from easy access from the outside. However, a determined and skilled computer hacker could probably find a way to gain access to my computer so I do not use any kind of client data management or comprehensive database system. Client credit card numbers are kept offline.
I do not retain client data on any web based computer system that is stored on any third party computer system, except for backup purposes.