I'm not really in favor of a Spyderco sub-forum, and here's why:
The Busse forum works, because it's for hogs; they know what they want to buy, and they know the re-sell market is there if they want to take advantage of it.
Spyderco users are, for the most part, general knife enthusiasts. Certainly there is a population of us who only buy/collect Spydies, but that's not the case for the majority. The sub-forum would not be bigger, it would be smaller. Where an EDC spydie might get 50 pageviews on the general FS forum, it might only get 10 on a sub-forum. So long as it's the right knife at the right price you won't have any trouble selling to the Spyderco community, but you're weeding out the newbs and generalists who might buy a knife they know little/nothing about.
The multi-brand threads are what makes this a bad idea, IMHO. You're splitting up sales of similar knives, and making it more difficult for sellers to keep track of what they've listed. For Busse, it makes sense (Hogs only want INFI anyway); for Buck collectors, it makes sense (they're looking to buy and to sell collectibles); for me as a Spyderco user, the collectibility is second to EDC practicality--I like looking at multi-brand sale threads, even though I'll more consistently buy Spydercos. I'm looking through threads will all kinds of EDC knives anyway, why wouldn't I want to look for EDC Spydercos in the same place? If you include spydies in multi-sale threads... well, what's the purpose of the sub-forum then?
Sure, it would be nice not to have to look at 15 threads before I see that there's a Dodo for sale; I think a more accessible search function would perform the same purpose, without reducing pageviews.
I'll continue to use the forums either way, I'm just sayin' it's not the most logical distinction to make (even if you can't stand Spyderco, you'll still see the knives in multi-brand sale threads.)
Perhaps a more useful distinction would be a forum for "users" and a forum for "collectors"? (Even there you'll get a lot of overlap--Where does a Sebenza go?)