Something definitely seems incomplete about the basis of "cannot be sold to someone else", because the users who registered on these sites (phpbb.com, my site, most sites) had no idea whom they were giving their data to in the first place. So the fact that "there is a new owner and operator of the forum" has changed what, exactly, with regard to expectations the user could have reasonably held? It's like saying your privacy has been violated, when in reality you've sitting in the middle of a public library the whole time.
I see language in the standard terms of service about "not disclosing information to third parties." But again, when the question of "whom is even running this forum to begin with" isn't something the user has proven or required disclosure of in the first place, how can you declare someone to be "a third party"? Meaning is "selling the forum" fundamentally any different at all from "we've simply brought on an additional admin." And some days or years later, the old founders just retire, or stop showing up. All the same information has been exposed to the new admin, but "without a sale."
It's not a legal definition, but what I expect from what I agreed to when registering on phpbb.com is that "you're not selling or using my information for some purpose other than running this forum." They are still upholding their end of the bargain, in my opinion, if they turned over phpbb.com to entirely new operators; sale or no sale. So long as this new owner isn't "selling or using my information for some purpose other than running this forum", I'm still in exactly the same position I was in before, and still have exactly the same expectations about using phpbb.com as I had before.
Maybe the words "this forum" is the key. The problem I could envision having with "a new owner" is that they want to change the subject of the board, and begin contacting members about subjects the users would have never joined a board for, never would have opted into, or never would have agreed to provide their contact information for. But that's not "a sale problem", because you can have exactly that same issue even if the board is still under control of the original founders. That's a "you got me to join this board, and then changed what it was about", regardless of operator.