I see it like this:
I have trade category/forum
All users, for starting new threads in this category/forum, must choose one checkbox "buy" or "sell"(they are going to sell something or buy something)
And I have, for example, money exchange category/forum
All users, for starting new threads in this category/forum, must choose at least one checkbox: "usd", "uah", "rub"
And add filters for categories/forums. When I am at the money exchange category, I can choose display only threads with prefixes "usd", or "usd" and "uah".
It makes some categories more readeable. If I want to see all, I will see all. If I am interesting only in some threads, I will see only some threads. It's bad way to make some forums for only "usd" or something other, because of maybe somebody make thread with all checkbox and going to work with "usd", "uah", "rub"