First thing's first: when making the attempt to import an automobile into the US, do not assume that the salesman spewing unintelligible legal jargon has the means of getting the job done legally. Neither should you assume that he's in full disclosure of the terms behind the vehicle's importation. Motive's article covers all the standard legal means, so if someone is selling you a process you haven't heard about before, run. Unless over 25 years old, in order to be federally legalized for street use a vehicle must comply with DOT standards for its year of production. It must also meet EPA standards, which means if the vehicle was manufactured during or after 1996, a complex ODB II diagnostic and sensor system is required. Very few registered importers have built reputations for successfully modifying imports for compliance of DOT requirements and there is an extremely short list of independent commercial importers available to install EPA demanded components with resulting proof. Both entities come at an enormous expense, often multiple times the vehicle's worth. The show or display exemption is an exception to the DOT requirements, but the qualification requirements are stringent. Plus, almost all exemptions only allow for importation on a temporary basis; when the bond period expires the vehicle must be exported or crushed. On top of that, working around EPA and California regulations is a whole different can of worms.
Once the information outlined by the Motive piece is digested, a lot of firsthand knowledge can be acquired from the Skyline GT-R/GT-S forum at FreshAlloy.com. The owners of the few federally legalized Nissan Skyline GT-Rs have seen, heard and shot down every conceivable importing scam. Posters look out for shady internet sales operations and they are also quick to point out fraudulent sales advertisements. They are very particular about the distinction between grey market and black market vehicles as well. There are automobiles that make it through customs and even end up registered on roadways, but are illegal in the eyes of the federal government therefore subject to impound.
[Source: Motive Magazine]