and I have been a 'victim' twice of this shocking practice.
I call it ram-spam-sell-offs - basiclly what it entails is that a forum owner will invite (spam) a hell of a lot of people from other forums, effectively harvesting human beings like cattle.
For what reason?
To build up the numbers on his/her forum and then suddenly and without any notification sell the forum.
IMHO this is an insidious and somewhat dishonest practice and a betrayal of people's trust... worse still, when people mass-spam another forum for that reason.
An example, something I saw on a administrators' forum, this member proposed buying a smaller forum, invite them and even redirect them to their main forum, then sell the main forum.
Being at the worse end of this twice, I can tell you it really annoys the crap out of me.