No board of public utilities created under the terms of this subchapter shall ever have the right to sell, mortgage, or create any lien whatsoever upon any of the plants under its jurisdiction, except that the board, from time to time, may sell and dispose of any machinery or equipment not necessary to the maintenance or operation of the plants, and, in the purchase of any equipment for the plants, the seller may retain title to or a vendor's lien upon the property purchased from him by the board.