I've never bought from BMMs myself . I imagine you know that any S&W 59 series mag will work in the Camp 9. Looking thru the site you mention I see that many of the mags listed don't include the brand or maker's name. My guess is that many of these are Promag, Triple K or even worse National Mag. Performance with some makers can be less than desired to completely unreliable. The prices listed for known brand items looks to be typical at best.
I'd recommend sticking with factory Smith or Mecgar mags. Greg Cote LLC is one top notch internet supplier and there are others also such as Midway, Topgun Supply, Brownells etc.