you can only post in this area on invites if you are giving them away, not trade. most of the sites that you are registered to specifically say that you cant use your invites to trade or sell them and if they find out you are you are, banned from there system. at least that was to my understanding. and anyone can correct me if im wrong on this.
Free Stuff means just that....not trade for another invite. Short answer to your question is yes, it's against the rules and there is no other forum where it is allowed.