Is there a difference in accuracy between having blood pressure taken by a professional and a machine?

It depends. Usually a person is more accurate than the machine, especially if no one is paid to check the machine as it ages.
Possibly. It is important to be certain that the blood pressure is accurately taken. If a patient is checking his or her own blood pressure, i commonly will have that patient bring the machine in so that i can compare the readings to make sure that the home blood pressures are accurate. Sometimes they are and sometimes they are not.