Is there a key for all the worlds of the spiral that you can buy? I have quests in Marleybone, but i'm stuck on Krokotopia...
You can not buy the key, you must complete the last area of Krokotopia to enter MarleyBone the correct way.
There are 2 reasons why you would have quests in MarleyBone out of you completed order of the worlds, you have a school spell that requires you to go there or you helped someone there and picked up an available side quest you came upon at the time.
If it is for a spell from you school a lot of wizards ask another wizard who has access to the other world for a teleport there and I don't think that is cheating per say. I don't think spiral hopping is the best idea so I do not contribute to that. If you do that you are only hurting your self not following your quests in order since your wizard is not quite prepared for the next world so either you have to have help with every fight or you struggle and die a lot.
You cannot simply buy a spiral key! What would be the point of that? You must earn each spiral key by going through the story line of the game and making your way through progressively harder worlds. To get the MarleyBone key you must beat Krokopatra in Krokotopia and then you shall be awarded the Spiral key to Marleybone. Then once you beat Meowiarity, you shall be given the key to MooShu, and so on.