Section 6. In a proceeding under either of the two preceding sections, if the person so employed does not make an actual transaction relieving him from liability under section four, the fact that the seller or the person employing another to sell for his account did not own the securities or commodities at the time of the contract of sale or of the giving of the order to sell, and the fact that settlements were made without completion of the purchase or sale of the securities and commodities bought or sold or ordered to be bought or sold, shall each be prima facie evidence that within the meaning of section four there was an intention that there should be no actual purchase or sale, and that there was reasonable cause to believe that said intention existed; and the parties liable to an action under said section shall be jointly and severally liable.