If A is bargaining with B over the sale of his house, and if A were given the privileges of a modern labor union, he would be able (1) to conspire with all other owners of houses not to make any alternative offer to B, using violence or the threat of violence if necessary to prevent them, (2) to deprive B himself of access to any alternative offers, (3) to surround the house of B and cut off all deliveries, including food (except by parcel post), (4) to stop all movement from B’s house, so that if he were for instance a doctor he could not sell his services and make a living, and (5) to institute a boycott of B’s business. All of these privileges, if he were capable of carrying them out, would no doubt strengthen A’s position. But they would not be regarded by anyone as part of “bargaining” – unless A were a labor union.
(I first learned of this useful monograph through F.A. Hayek’s quotation of the above in The Constitution of Liberty.)