Only the domestic production of new such magazines for sale to civilians was prohibited.
"Under the assault weapons ban, it was illegal to manufacture or sell new high-capacity magazines, defined as those that hold more than 10 rounds."
"Stores could legally only sell used high-capacity magazines at that time, and new magazines could not be manufactured."
http://www.salon.com/news/politics/war_room/2011/01/09/giffords_shooting_assault_weapons_ban It should also be noted that the importation of high capacity magazines for sale to civilians was still legal during that time.
"Second, the legislation would close a loophole in the 1994 law, which
prohibits the domestic manufacture of high-capacity ammunition
magazines, but allows foreign companies to continue sending them to
this country by the millions."
http://feinstein.senate.gov/booklets/Assault_Book.pdf