Section 6. In an action for the recovery of an outstanding debt or claim sold or assigned by an executor or administrator under a license of the probate court costs shall be recovered by or against the plaintiff but not against the executor or administrator. Any such action which is not governed by the Massachusetts Rules of Civil Procedure or the District-Municipal Courts Rules of Civil Procedure shall be brought in the names of the purchaser or assignee, and the fact of the sale shall be set forth in the pleadings. The defendant may avail himself of any defense which would have been open to him upon an action or proceeding brought by an executor or administrator.