Some private BT sites will ban users who trade invites publicly. They keep an eye on public forums such as this and when they see someone offering an invite, they ban that user.
Also, some sites ban members for inviting people who are already banned from their service. Please do not ask for an invite to one of these sites if you have been banned.
On this note, anyone falling victim to this (being banned for inviting a banned user) should PM a mod with details (including the notification you get). We don't want this sort of thing bringing down our rep.
This thread is intended to compile a list of these sites so please reply with details if you've been banned for trading or offering invites to their service. Also reply if you know a private BT service that has this sort of policy, posting a relevant link.
The thread will periodically be condensed into this first post. I other words, BT sites will be added to this post and the original post trashed.
This will make it easy to see where trading is going to get you in trouble.
Sites that ban users for trading their invites:
Sites that ban users for inviting previously banned users: