Retailer's tax exemption for new truck equipment distributor?

​​​​​​​We are a new truck equipment distributorship, and we are establishing relationships with body manufacturers. Several manufacturers of taxable truck bodies have advised us that their sales of the bodies to us will be a taxable sale under Internal Revenue Code (IRC) Section 4051 unless we provide them with some kind of certificate. We thought only retail sales were subject to tax under IRC Section 4051. Are the manufacturers correct and if so, what type of certificate should we provide?