FinCEN requirements, by my understanding, are concerned primarily with businesses which transfer money on behalf of other parties. If Alice transfers money to Betty to pay her for goods or services, neither Alice nor Betty is an MSB from FinCEN's point of view. But if you add an intermediary (let's call her Catherine) who takes money from Alice and gives it to Betty, and does that sort of thing commerically, then Catherine is an MSB and subject to regulation (because if Catherine wanted, and didn't have anyone watching, it'd be easy for her to hide questionable money in the cloud of transfers which she manages).