It doesn't have a rare requirement. and it doesn't seem to have a lock. I have farmed / bought all of the items needed for completion. I'm just afraid that pressing that "turn in" button will send my account between the banhammer and a hard place. So, what I'm really asking is, if I press that button, what will happen? And if there is NO lock on that quest, and I get the item, what happens? Nothing? Everything bad? Something kinda bad? That's my question I'm trying to ask.
I'm almost certain that you won't be banned unless a Staff member specially takes a look at your account which in most cases they won't. There have been many cases of players purchasing rare items via shops that have been long gone and nothing has ever happened to them.
However I don't think you will able to turn in the quest. A quest that has gone rare CANNOT be turned in.