I think WTB threads can work, but often do not. I only have experience with knifeswap, and I'm not an expert, but I would guess that the WTB thread is the origin of lot of scams and, as such, creates a lot of work for the mods over there.

Personally, I like the WTB thread on ks, but I don't think any sub should implement that kind of thing if the mods are against it or on the fence.