It shall be the duty of every person engaging or continuing in this state in any business for which a privilege tax is imposed by this division, to keep and preserve suitable records of the gross sales, gross proceeds of sales, and gross receipts or gross receipts of sales of such business and such other books or accounts as may be necessary to determine the amount of tax for which he is liable, under the provisions of this division. It shall be the duty of every person to keep and preserve all invoices of goods, wares, and merchandise purchased, for resale or otherwise, and all such books, invoices, and other records shall be open for examination by the Department of Revenue or its duly authorized agent. Any person selling both at wholesale and retail shall keep his books so as to show separately the gross proceeds of wholesale sales and the gross proceeds of retail sales. All sales shall be subject to tax in the absence of such separate records.