All forums have gone quiet in comparison to the last few years
http://p8ntballer-forums.com/threads/is-it-always-this-quiet-round-here.152392/page-6
Facebook has taken over the general chat, and picked up a few niche subjects but is useless for holding knowledge.
P8ntballer & ukscenario are the two strongest uk forums.
Uks has slowed down but is fairly stable - it looks a lot worse then it is with the 'new posts' search which restricts the results
P8ntballer has recently picked up coincidentally with the rule change on new members and access to the classifieds.
For selling this forum has the biggest throughput, uks has a few sales, the other uk forums also but have also slowed down.
There are a few Facebook pages & groups for sales, either like minded communities based on an interest or just sales pages.
However for sales these can have the disadvantage of Facebook that individual sales can quickly disappear