Federal regulations make it's very illegal to take a vehicle apart, import it as spare parts and re assemble it for the purpose of on road use in the USA.
Some States allow you to make a vehicle out of parts and title it. But States do not regulate imports from foreign countries the Federal Government does. We are dealing with two different sets of regulations here.
Green's point illustrates the seperation of Federal and State regulations. The State doesn't care if a vehicle is legally imported or not. If they can run the VIN then most States will issue a Title.
In the case of home built trailers a State will issue a VIN. But it streaches the point when the home built trailer is in fact a previously manufactured trailer or re assembled trailer.
Is it worth importing a trailer illegally?
If you get caught you either have to export the trailer or pay to have it destroyed. In addition you have signed a legal document stating that the vehicle you are importing meets all Federal safety standards and regulations, so look at a fine as a minimum .
Is it worth importing a used military trailer financially?
Sankey trailers from Witham
http://www.witham-sv.com/registered.php cost around $500.00 each. Crating $250.00, inland shipping to port $250.00, Less Than Container Load shipping $650.00, Customs clearance, bond and duty $250.00 a Total of $1900.00 plus inland trucking to the US destination.