It's not really that confusing:
If you get your mmsi # from the FCC, the # stays with the boat, period. You can't transfer it. The only way that # can be used again, is if you cancel the registration first. It is transferable to another owner of that boat, but they would probably have to pay a fee for this. Handheld radios that are used with that boat can be programmed with the same # but they need to stay with the that boat.
The mmsi #s you get from BoatUS can be transferred to the new owner if you sell the boat. You can also transfer the # to another boat if you move the radio and update your boat info with them. The owner of the mmsi # first has to release the # to BoatUS to make the transfer to another owner.
That said, there's nothing in the system that would prevent someone from selling a used radio with an existing mmsi # programmed into it. That's up to the seller or the buyer to make sure the existing # gets wiped before the radio is used.