This is an olde, but it still applies today.
With the ability to do character transfers on different servers, there needs to be a similar service for accounts.
There are numerous ways to make this safe with little no RMT hassle.
1. Charge for the service
2. Put a time limit on how many character transfers an account can partake in and the amount of characters to be transferred, i.e. 1 character a month.
3. Restrict it to accounts with the same credit card billing information. This almost ensures that it isn't RMT as they wouldn't have your billing address.
With FFXIV out, there is no reason to hold anything back from XI that can possibly enhance game play in a safe manner.