Tulunk Village

Does it even show up if you try to do it? Once I tried doing a key in a mod evolution, but the only way it would even show up is if I used a GameShark code to get lots of keys. Even if I got a key legitimately and tried to use it in a mod evolution (this is possible in that hideout in Panses Village -- just get the key and exit), the key wouldn't show up.

Where does it not show up?

If you mean it does not show up in the shop, maybe its because plot items are made such to match the objective (eg key match door) Until the door is unlocked, the item cannot be used for other purposes aka reform.

If so then items like key and toy can only be used in reform once you finish the story.

Just guessing.

One example is the hideout where Saiope is (the blue colored one with lots of mazes). It is possible to get the key and then leave it. And since you can leave, you can go to any mart. However, if you try to do this, you'll notice that the key is missing, even though you have one. I'm not sure why this happens, yet if you use a GameShark code to get keys, THEN they show up.

But when you use the key for the door, isn't it gone forever?

Did you try the item cheat before using the key for the door?