There are federal standards that must be met. If one of your trailer kits came apart (even though it was the customers fault) you would have to defend yourself in a lawsuit. If you failed to follow the federal standards (regardless of the state laws) and someone were killed, you could find yourself behind bars. Better look into a $1,000,000 manufacturers insurance policy.
Beyond that, you would need to find a parts wholesaler relatively close by, preferably two. Most will deliver at no charge. If not the inbound freight would kill your competitiveness. The reason for two suppliers is twofold. The competition between two suppliers will push your costs down and if one supplier backorders something you have a backup. Wholesalers like Redneck can be very competitive, but costs will depend on volume. Most suppliers will give you the benefit of doubt when it comes to forecasting to establish your discount, but be realistic with your forecast.
Outbound freight will become a major issue with trailer parts. There are alot of freight discounts out there, but you might have to join a builders or manufacturers association to get them. Make your kit as dense or compact as possible to get a low freight class. The freight class system is a very arcane involved system, but basically the lower the ability for the trucker to damage the goods, the lower the freight class. Shoot for a 55 or 60.
Good Luck
