While attempting to submit an MCF order, I started encountering the following response from the SP-API:

Value NotEligibleForExport for parameter Item is invalid.

A few articles mentioned that this was caused by not using the right marketplace ID but adding debug statements showed this was not true.

The solution ended up being pretty simple, I was using the wrong key for this ID.

"marketplace": "'.$this->marketplaceId.'"

Should have been:

"marketplaceId": "'.$this->marketplaceId.'"