You can try flippa.com - it's pretty much the universal website buying/selling place.
Having done it myself in the past the only suggestion I'd have is check every single claimed statistic is real before buying. Seriously, it's way too easy for people to fudge statistics on Flippa - never trust screenshots claiming revenue and traffic.
The last time I checked Flippa (days ago) there were about 25 listings for forums.
One of the issues I see with the ones that are selling is that most of them are dead forums.
There is one that boasts a lot of posts. But in further digging, the majority of posts are hidden in a sub-forum and is full of old news articles brought in from rss feeds.
Some of the other ones I looked at you could tell that the forum has been dead for a while even if there are a few new posts.
Check who is online for several days to see if there's activity. Check dates on a lot of the posts. Check if the stats match what you are seeing and how they add up and where most of the posts are made. Check the members list and see when the last time the top posters logged in.
Other places for buying forums:
And domain auction websites.
Cool to see. The main thing I am missing is keywords / tags and the possibility to watch such tag. Rather than browse advertisements every once in a while (which I dont do often anyway) its much easier to just get notified when a site in your niche has been posted. Flippa has this. DP should too.
Because there are a lot dead or websites or websites that had serious problems on there. Like a penalty from google or something. Or at lest it looks like that.
But why can't Flippa request better proof before they can post?
That's right that's the way to do it to filter all the crap.