If this is truly a farm structure, located on a farm, then no permit is required. But if hay being sold isn't produced by owner, it's a retail structure. The way around this is to back doors of trailers out from under structure when selling. Your truss manufacter will furnish SEALED drawings. Due to width (escess height when transporting,) you may have to have trusses made in two sections, fastened together over a center beam with related posts. Post will have to be set several feet into the ground to resist wind racking. Posts, beams, and trusses will have to be tied together with what local conditions require. Eastern NC has a much higher wind zone than central NC. MBCI in Chester VA has a rolling mill for roof sheets. Back in the eighties and ninties, they had a regular truck coming to Raleigh area once a week. 50% on order, and balance BEFORE truck is unloaded, unless credit is established prior. Panel screws can be bought from ATlas Bolt and Screw. You can increase wind lifting resistance by using "L" shaped metal (Hamlin sheet Metal in Garner) on top of framing members. To gain clearance, you can hang trusses off side of LVL beams. Trusses can be ordered with extended top chord to furnish over hang. Can trailers be pulled or backed straight into proposed structure. If not post spacing will have to be wider. Outside of each post, set a bumper pipe. My first choice would be a 15" RCP, set about a foot into the ground, filled with concrete. Forget 4" steel pipes as a trailer will wipe them out quickly, with support post to follow. If you go with steel framing, then you will need a concrete slab to resist the outward thrust of the support posts. The Morton Building people specialize in these types of structures.