A lot of these invites can be had for free from http://www.reddit.com/r/Invites/ or http://www.reddit.com/r/opensignups

Got an ebook.farm invite in less than 24 hours, for free.

Moreover, *selling* invites is considered a dick move by almost every private community, and will get you banned.

Man, people will sell you your own damn shirt as long as someone is dumb enough to pay.