The title has been saying "Selling/buying is forbidden" since it was created, signatures are also part of OtLand so it obviously applies there too unless the opposite is stated.
You would have learned it the easy way, but you made it hard for yourself.
First, your signature was removed multiple (at least 5 times) times by a moderator and you kept adding back the selling stuff. After it was removed a few times, you created a thread asking about it and I told you that it was not allowed and I removed it. You replied and said "Stop removing my signature!!" as you added your signature back. After viewing the thread again I saw that you put your signature back with the selling stuff, so again I removed it, posted that it wasn't allowed AGAIN, and I closed the thread... What do you do next? You're being so smart and open the thread with the privilege I would have expected you to be more careful with (because you applied for moderator), and you put your signature back and you claim that I have told you that you were allowed (which I never have told you, just went through all my sent messages to you and couldn't find me saying that).
That was too much, so I removed your privileges to open/close threads, gave you an infraction, specified the reason well, and removed your signature. A few minutes later you put your signature back with the selling stuff, and I gave you your final warning (second infraction) and removed your signature again. After that I wasn't even on the forum, but I heard from the staff member that banned you that you had put your signature back with the selling stuff, and that is why you got banned.