It was interesting to read that the price does not include the existing users and data. Which would in my eyes, possibly be the most valuable part of purchasing a website like this. You could pay for or develop something similar for much less that the starting price here.
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.
I think it's actually a pretty good thing to do, both for the seller and the users.
If companies didn't sell on the account details of their existing userbase these users would effectively be frozen out of any future site development/improvements (unless they re-entered all their data).
And since acquiring users is hard, from the seller's point of view an active userbase is probably one of the site's biggest assets (and also a validation that the site has some potential to it).
This is what i meant that the user data is valuable otherwise you would have frozen out the existing users and have a web platform you would have to build up a userbase on again, which if this was the case then you would just start your own project as you are starting from scratch. The only upside is existing traffic to the website. Existing users who turn up one day to realise the content they have added is now gone would be a disaster for the user, then asking them to sign back up to recover that data is another unnecessary step.
