Section 11-15-6 - Vendor must show sale or service exempt; presumption(a) The burden of proving that a sale or service was exempt from the tax shall be upon the vendor, unless the vendor takes from the purchaser an exemption certificate signed by and bearing the address of the purchaser and setting forth the reason for the exemption and substantially in the form prescribed by the tax commissioner.(b) To prevent evasion, it is presumed that all sales and services are subject to the tax until the contrary is clearly established.