Wis. Stat. § 77.57

If a purchaser certifies in writing to a seller that the tangible personal property or items, property, or goods under s. 77.52 (1) (b), (c), or (d) purchased will be used in a manner or for a purpose entitling the seller to regard the sales price from the sale as exempted by this subchapter from the computation of the amount of the sales tax and uses the property or items, property, or goods under s. 77.52 (1) (b), (c), or (d) 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 or items, property, or goods under s. 77.52 (1) (b), (c), or (d) to the purchaser.