The GLBA privacy provisions' only accomplishment is that they govern the content of a written notice which must be given to consumers. They neither prohibit nor authorize disclosures of customer information. They only give the consumer a right to know that certain disclosures are made and a right to opt out of some of them. The fact that the regulation does not address what a previous affiliate can or cannot do with previously acquired customer information is not an indication that the affiliate may continue to use it.
This should have been addressed in the terms of the agreement under which your agency was sold. If you use the information, it is your prior parent organization's opinion about whether it is appropriate that matters. Your attorney needs to be involved in any decision on your part to continue to use the information.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.