Each certificate which any collector of school taxes makes of delinquent school taxes, levied upon real estate upon which there is no personal property from which the same can be collected, shall be accompanied by the affidavit of the tax collector that, after the proper efforts, he could not find sufficient personal property out of which the taxes or any part thereof could be made or collected as provided by law. Such affidavit shall be conclusive as to that fact in so far as it may affect the lien of said taxes or the title of a purchaser at a sheriff's sale thereunder. In the event any such collector shall make any wilfully false return he shall be liable therefor to any person or persons injured thereby.