Why? I don't read that section, but I feel like most of the posts there should be fairly substantial and contribute to a person's "record".
Investor based games often have threads which the OP of such threads encourage "players" to both post when they "invest" and when they have their "investments" returned (they are essentially getting people to bump their thread for them).
I would agree with the auctions sub not counting, especially posts that are not an OP considering that some auctions are sig spam heaven in that the minimum bid increment is sometimes ridiculously small like 1 satoshi.
I am not so sure about the other marketplace sections though, as even though the above types are posts are generally useless and made by sig spammers, there is somewhat of an incentive for people to warn others about shady dealings, and when things don't (or do) go smoothly if such posts "count" towards post/activity count.
I suspect the goal behind this is to reduce signature spam throughout the forum, and if this is a true statement then I would suggest that posts throughout the forum count towards a user's post count, but certain sections do not count as posting towards posting in an "activity period". The reason for this is that if a particular section did not count towards a user's post count, but a signature campaign wanted to encourage participants to post in such section then the campaign manager can manually count posts in that thread from a user's post history -- the opposite is true today in that all posts count towards post history, but for example campaigns want to discourage people from making low quality posts in "
off topic" so they manually exclude posts made in
off topic.
I would suggest finding some boards that should not count towards activity, as I would theorize that the worse of signature spammers both farm accounts (eg collecting potential activity by collecting activity posts by making a single post every two weeks), and spam the forum with paid signatures.
Another potential solution would be to make it so you need more then a single post made in a two week period in order to collect the (potential) activity points. It could either be setup so that you need to make at least 4 (or some other number equal to or less then 14) posts in order to collect the 14 activity posts, or so that you both need to make at least 2 (or some other number equal to or less then 14) posts
and be online at least 2 hours (or some other amount of time) during that activity period in order to collect the activity points. Yes time online can easily be faked, however it is at least a small hurdle for people who are spamming the forum in order to collect potential activity points.