Sales and Use tax go hand in hand. He provides you a resale license, you pass off the burden of collecting the sales tax to the buyer, end of story. If he uses his resale license to avoid paying the sales tax, but opts to use the product, then the buyer is responsible to pay the sales tax in the form of a use tax. If the guy says he's going to use the graphics to help sell the trailer, are you still going to charge him sales tax?
Yes, I would charge sales tax if he's using the graphics to sell the trailer. He's still the end user.
Is every sign maker so dense? There should be no question or argument about this.
Try not withholding tax on your employees some day and telling the IRS that it's the employee's responsibility to pay it voluntarily.