If you buy and sell gun parts on ebay, you may have noticed disclaimers about ITAR regulations. Without getting into too much legal jargon, I will provide a general idea of what ITAR is.

The International Traffic and Arms Regulations (ITAR) is a collection of regulations that basically states you cannot "export" certain types of items without an export license. One purpose is to restrict the amount of domestic arms and arm/products from being obtained by international hands from US civilians. Manufacturers of specific equipment can export to foreign allied military/government.
The criteria covers most "military-grade" equipment. Here is a sample of some items which are restricted from export by ITAR:

• .50 caliber firearms/parts (example: Barrett rifles/parts/ammo)
• Military-grade rifle scopes (example: EOTech holographics, Trijicon ACOG)
• Lights/lasers (example: Surefire weapon lights,
• Optic mounts (example: Larue mounts)
• Night vision
• Silencers
• Flash suppressors (example: AAC 51T blackout flash hider)

There are some strange things listed such as "ammo linking machines" and other tooling which can be used to produce munitions. Consumer grade reloading presses ARE NOT restricted by ITAR.
See more details regarding ITAR.