24hourcampfire is a great site and the Classifieds section is incredibly active. I see no reason to create sub-categories. In fact, I find sites with sub-categories are a PIA.
I suspect most of us enjoy looking at knives, firearms, optics etc. and don't logon to effect a specific search.
While I've never posted a For Sale post and likely never will, having an ad in proximity to the other ads vastly increases exposure and likelihood of a sale.
Even if site moderators were willing to create a knife sub-category, I would urge them not to do it.
NRA Endowment Member Firearms Accumulator Proud Trumpster!!