A NTSB report says David McElroy of Orient and his mechanic "represented" to the potential buyers that the plane was airworthy when it wasn't.
The report found the aircraft had a faulty tachometer and a lack of engine power during a test flight three days before the crash.
A tachometer measures engine revolutions.
The report says the mechanic never "signed off" on the inspection and didn't restore the plane to service.
Erik and Jane Unhjem were interested in buying the plane. She died in the crash; her husband survived.