Guys, it is illegal, period, for any US citizen to buy and ship you what are considered "essential" gun parts. This includes stocks, any and all internals, including bolts, barrels, or any other part of the gun.
The ONLY exception to this is if the citizen, acting as a business or not, has a valid export liscence. Some parts require US state department approval for export, such as bolts, barrels, receivers, whole firearms, complete triggers and any item deemed an essential part that exceeds a certain value. This value is 100 to 300.00 dpending who in the US you talk to.
Sights, scope mounts, bipods, grips, and a few other items deemed by US state department to be NON essential firearm parts, do not fall under a maximum value and are NOT subject to state department approval.... BUT AGAIN, the sender/shipper/seller and in most cases the manufacturer MUST be liscenced for export. If the item's manufacturer or distributor at origin does not have the item export liscenced, ANY U.S. Citizen acting alone or as a business, is comitting an offense if they ship the item to you.
It is really, that simple.