Answered Pokemon that knows unbuyable attacks

[R3Z 0]

I bought a Dark Spiritomb from an auction and it has the Will-O-Wisp attack which is not in a Spiritomb's learnt attacks nor in the purchasable attacks. So is there any way to teach a Pokemon a move which is not in it's purchasable attacks?

[Patrick 2,015]

No there isn't. If a Pokémon knows a move it cannot currently learn then it's because it had that move from an older move set either base or purchasable.