https://www.faa.gov/about/office_org/headquarters_
Post# of 41413
Background
Part 125 was issued to establish a uniform set of certification and operational rules for large airplanes having a seating capacity of 20 or more passengers or a maximum payload capacity of 6,000 pounds or more, when common carriage is not involved. These rules substantially upgrade the level of safety applicable to large airplanes formerly operated under Part 91.
Note the phrase: when common carriage is not involved. I don't think it would be the appropriate cert. for this reason.
jz