My approach would be to inform current users via email or some login notice lightbox about the situation, along the lines of:
"This site is in the process of being transferred to new ownership. We value your privacy, and will not provide any of your data to the new owner without your explicit permission. If you don't want to continue using the service following the transfer, you don't have to do anything (but you can download a snapshot of your own personal data from <link>). If you do want to continue using it, tick this box"
The only problem I can see with that is that most users should rightfully refuse until they know what the new owners plan on using it for, so you need to have a sale lined up and a new privacy/data use policy drawn up before you can do that.
Perhaps you'd keep a copy of the database yourself, and then once the sale happens and terms are hashed out, send the mail to everyone, and deliver a copy of the filtered opt-in database to the new owners.
"This site is in the process of being transferred to new ownership. We value your privacy, and will not provide any of your data to the new owner without your explicit permission. If you don't want to continue using the service following the transfer, you don't have to do anything (but you can download a snapshot of your own personal data from <link>). If you do want to continue using it, tick this box"
The only problem I can see with that is that most users should rightfully refuse until they know what the new owners plan on using it for, so you need to have a sale lined up and a new privacy/data use policy drawn up before you can do that.
Perhaps you'd keep a copy of the database yourself, and then once the sale happens and terms are hashed out, send the mail to everyone, and deliver a copy of the filtered opt-in database to the new owners.