Selling invites is allowed, but there are some restrictions in order to prevent people from selling accounts and invites obtained through hacking. In order to sell invites or accounts to invite-only sites, you must:
Have some history here, with more history being required for trades of invites/accounts in bulk. Or,
Have substantial history on reputable invite trading forums such as torrentinvites.org which prohibit selling hacked accounts. Or,
Be vouched for by someone from one of the above two categories.
Most importantly: selling service accounts wherein the registered user is granted provisional access is RISKY - it is safer to trade what is owned and tradeable/marketable.