Answer: a group boycott
Step-by-step explanation:
A group boycott refers to the commercially motivated action which are taken by a group of companies agsinst their suppliers it competitors. A group boycott can be used for the implementation of an illegal price-fixing agreement. In the United States, it should be noted that a group boycott is seen to be illegal as it violate the Sherman Antitrust Act.
Based on the information given in the question, since the other brokerages in the city decide to pull all their listing from the MLS and create their own listing database, then this is referred to as a group boycott.