All sales or conveyances of lands, tenements or hereditaments, which shall hereafter be made by virtue of any letters or powers of attorney or agency, duly executed, which do or shall expressly give power to sell lands or other estates, and be certified to have been proved as aforesaid, or shall be proved in this province before any justice of the peace, by one or more of the witnesses thereto, shall be good and effectual in law, to all intents, constructions and purposes whatsoever, as if the said constituent or constituents had by their own deeds, bargains and sales, actually and really sold and conveyed the same.