An estimate of value.
An appraisal can only be performed by a licensed appraiser certified in the state where the property is located.
Broker's price opinions are not opinions of value, typically are not accepted by lending institutions, state and federal taxing authorities, and are definitely not defensible in court.
There are many reasons. The most common reason is that the assessed value in Massachusetts is based on the value as of January first of the previous year. Appraised value is current.
In most situations, the appraiser is hired by the lender (bank or mortgage company), to protect them from paying too much money for the property. While you have paid the bank, the bank is the client, and they are the entity to whom the appraiser owes his professionalism. Moreover, the appraiser is bound by state and federal laws, universal standards of professional appraisal practice (USPAP) and the ethical standards of his / her state or national affiliation (MBREA, Appraisal Institute).