(xv)The vendor shall not knowingly endorse on any impressed sheet sold the name of any person other than the actual purchaser or the person on whose behalf the stamps is being purchased, deliver any stamps sold to any person other than the person whose name is endorsed thereon as that of the purchaser. The vendor shall not also retain in his possession for more than seven days any endorsed stamp entered as sold in his register of which the vendee fails to take delivery. In case of such failure he shall, on the expiry of seven days, forward the stamp to the Collector for cancellation and refund of its value or issue of another stamp in exchange thereof.