If the seller is your parent, grandparent, or sibling and already paid sales tax on the truck when it was purchased, you don't have to pay sales tax when you buy it. However, you must complete a family sale exemption certificate. (The exemption also applies if the seller is a step-relative still related to you by marriage. But, it doesn't apply if the seller is a cousin, aunt, uncle, or foster child.) To claim the family sale exemption, you must fill out Idaho Sales Tax Exemption Certificate, Form ST-133, Section I, and give the completed form to the clerk when you title and register the vehicle.