What is the make inoperative prohibition on used vehicles?

We are an upfitter, and one of our local dealers sold some used pickup trucks to a customer and sent them to our business for service body installations without consulting us or the OEM for pickup box removal guidance. After reviewing OEM materials for the make/model/year with the body manufacturer for whom we distribute, we determined none of the vehicles were eligible for box removal and returned the trucks to the dealer. The dealer is upset we would not install the service bodies and claims the OEM information isn’t valid because the vehicles are used. While we have long taken the position that we don’t perform box removals on models the OEMs do not represent for new models, the OEM materials do not seem to address used vehicles. Are there any applicable used vehicle regulations?