I'm a member of "The Brass Bank" on FB.  Nice bunch of guys for the most part, but you have to be accepted as a member of the group via request.  I've purchased some brass there... but just so everyone knows, 30-40 brass is a very rare commodity there.  Most of the people in that group are selling brass that they pick up at ranges.  Some good deals... and some not-so-good deals!  I'm betting that most shooters save their 30-40, which is why it just doesn't come up much, IMHO.