Auctions (https://bitcointalk.org/index.php?board=73.0), since bid posts contribute nothing. Maybe even include all the "listing" sections such as goods, services, currency exchange.
Auctions is a good choice. But, goods, services & currency exchange have lots of constructive discussions.
Like what? What's pretty much there:
bumps
"be sure to use escrow"
"scammer!!!1!"
"sale went smoothly"
questions related to transaction (who goes first, escrow, counter offers, etc.)
people asking for vouch copies (in digital goods)
sig spammers asking questions but have no intentions of buying
Can you write a program to identify all those posts and exclude them in the post or the activity count?