It'd be similar to how the pages for towns work -- each town is supposed to have a wiki page already, though from what I've seen that isn't really enforced. For shops, it'd be highly useable -- certainly more efficient than a forum. Shops don't need a place for people to spam about wanting them to stock X item or lower Y price -- they need a place to advertise and be easily found.
Granted, that could be done using the forums -- I definitely see what you're getting at. But the wiki is far more navigable, would allow for the shops to be ordered in a list (rather than just being constantly bumped as they get past the first page of the forum thread list), and would be vastly more readable. Why go to the trouble of adding a whole new forum subcategory to do something the wiki could handle more efficiently? Is there something the forums would do better than the wiki in the case of shop threads?