You can not add an extra slot if you already have 12 slots on a server.
Here's a quote from the patch notes saying subscribers are LIMITED TO 12 CHARACTERS PER SERVER
If that is the case then its a very very bad piece of game design and very poor setup on the market. NO person should be able to buy an unlock on the market that is already unlocked for that character, or is not applicable.
I think we need CS clarification on this as the quote you reference is relatively clear, but does leave some room for interpretation.