What kind of headlamps are legal to use?
Retrofitting of an ENTIRE sealed OEM HID headlamp from a different vehicle keeping all original functions*) intact is legal in North America. Obviously, it must be mounted in the approximate same height and as solid as on the original vehicle. In a court case worst scenario, it will most likely will be used against you, but the right lawyer will be able to defend you.
*)Note that functions consists of low beam, high beam, markers, turn signals, amber side reflector, leveling system (if equipped), cleaning system (if equipped) and AFS system (if equipped).
All other kinds of retrofits are illegal, unless results from a testing lab can show otherwise. The cost of such test is around $10,000 and consists of various environmental, durability, material, aimiability, and so on. Even if you are willing to pay this amount, it is not granted that you will pass. Based on the report you will have to make design adjustments where it failed and go for another test. All in all, designing legal headlamps is not for the DYI mechanic.
Is it illegal to sell HID kits?
All HID kits containing an HID bulb intending to replace a halogen bulb is illegal to sell in North America. This is not a new regulation, its been there since way before HID kits came on the marked. It basically goes under regulations regarding non-compliant lighting. Equipment commercially made/sold in the US currently carries a fine of up to $10,000 per occurrence.
Is it illegal to import HID kits to the US?
Yes it is illegal. Unless equipment complies with current standard and is covered by a certification. This makes import a criminal act, you cannot blame the shipper or brokerage company. They can only advise you not doing it.
more info:
http://faq.auto.light.tripod.com/hl-hid-kits.htm
http://faq.auto.light.tripod.com/hl-upgrades.htm
http://faq.auto.light.tripod.com/hl-purchasing.htm
http://faq.auto.light.tripod.com/hl-general.htm
http://faq.auto.light.tripod.com/hl-current-regulations.htm
http://faq.auto.light.tripod.com/hl-systems.htm
http://faq.auto.light.tripod.com/headlight-menu1.htm