1. The burden of proving that a sale of tangible personal property, services, substances or things was not a sale at retail shall be upon the person who made the sale, except that with respect to sales, services, or transactions provided for in section 144.070. The seller shall obtain and maintain exemption certificates signed by the purchaser or his agent as evidence for any exempt sales claimed; provided, however, that before any administrative tribunal of this state, a seller may prove that sale is exempt from tax under this chapter in accordance with proof admissible under the applicable rules of evidence; except that when a purchaser has purchased tangible personal property or services sales tax free under a claim of exemption which is found to be improper, the director of revenue may collect the proper amount of tax, interest, additions to tax and penalty from the purchaser directly. Any tax, interest, additions to tax or penalty collected by the director from the purchaser shall be credited against the amount otherwise due from the seller on the purchases or sales where the exemption was claimed.

2. If the director of revenue is not satisfied with the return and payment of the tax made by any person, he is hereby authorized and empowered to make an additional assessment of tax due from such person, based upon the facts contained in the return or upon any information within his possession or that shall come into his possession.

3. The director of revenue shall give to the person written notice of such additional or revised assessment by certified or registered mail to the person at his or its last known address.