Selling is not always a bad thing though. A new person could mean massive improvements. Putting selling the forum under the umbrella of closing a forum i think is wrong.
By selling a forum the forum will still be there and things will normally carry on the same (if not better) in most cases. Usually it may be a bit bumpy to start off with. Sometimes though selling a forum can result in all the time and effort being wasted. Promotion-Source was a massive example of that. That was a masterclass is how not to run a forum and how not to manage taking over a new forum.
Closing down the forum is bad and a waste of time for everybody and annoy people. Selling the forum doesn't always result in that. The users who make forums and sell them and make new ones get a reputation and will ultimately lose out when they actually start a forum and want to keep running it.