It's not a rule or anything, more like common sense in my opinion. Unfortunately, some people will go out and collect spell caster keys by the hundreds and sell them on their vendors. I always put them on my vendors for free. I feel that this is disrespectful towards newer players, because if the player doesn't know how easy it is to get a free set, they could of saved a small (but NEEDED) amount of gold.
'Deadwerd' in game, 'Deadwerd' onstage.