Wis. Stat. § 77.57

If a purchaser certifies in writing to a seller that the property purchased will be used in a manner or for a purpose entitling the seller to regard the gross receipts from the sale as exempted by this subchapter from the computation of the amount of the sales tax and uses the property in some other manner or for some other purpose, the purchaser is liable for payment of the sales tax. The tax shall be measured by the sales price of the property to the purchaser, but if the taxable use first occurs more than 6 months after the sale to the purchaser, the purchaser may use as the measure of the tax either that sales price or the fair market value of the property at the time the taxable use first occurs.