Are both sentences OK?
They have made millions off of me.
They have made millions off because of me

Question

Answer

no

Answer

The first sentence is fine.  One implication is that someone/some company has made a lot of money selling you something.
The use of "off" in the second sentence is confusing and not natural speech.  
Without the "off" - "They have made millions because of me" has a different meaning from the first sentence.  It can imply that you did, or provided information, or you helped make something happen that resulted in creating millions for someone/some company.

Answer

They have made millions off of me.

The second choice is incoherent.

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