Oil!
The most important Chinese product imported to Europe in the late 1200's was silk.
Most Green tea is imported from China. Most Black tea is imported from Sri Lanka and Kenya.
Most cars now days are imported to the US. Any Japanese, Korean, or European cars are imported. Not all brands are imported, however. Some Hondas come from Canada, and some VW's come from Mexico. So "what is imported to the US?" is a very vague question.
Cheap imported goods
Assuming you mean to the US, I think most of the raw uranium is now imported, from Australia and Canada mainly, but the enrichment and fuel assembly manufacture is in the US.
Most are. Some are imported and badge engineered.
what is a restriction on the amount of a good that can be imported
Imported to.
If you mean slaves in the US, the answer is Africa.
embargo
Cost or availability.
40% of goods are imported from China to US