I don't see how there could be much of an increase in any RMT activity. The reasons you give to sell a character would be the same whether there was a transfer service or not.

Also, I was thinking if you simply allow a character to "separate" from an account, becoming the first character on a new account... there's no issue. By disallowing players to "merge" a collection of characters (from multiple accounts) on to one account, it would discourage RMT activity.

I don't think moving a character from one existing account to another would be the best way to do it, but I think some sort of service can be done without "back fires" nonetheless.