How did Beetlejuice get in the model?

Just trying to piece it together:
-There was a model of the town in the attic.
-There was a cemetery in the model.
-Beetlejuice had marker in that cemetery.

Beetlejuice either:
-transported himself, with a marker, to that cemetery,
or
-he was someone who died in that town a long time ago, and in the new, post-life world of the Maitlands, his name became Beetlejuice, instead of his original earthly name.

____________________
The story is king.

He resides in spirit world ordinarily (Juno knows him)
he detected somehow fresh customers were available
he magically created the grave and put on some hints who he was and what he did.
Business...its all business.

Better question...what do they use for money in the afterlife?

He has supernatural powers and can change his shape.

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