Trade with gaijin is obviously rare or non-existent during certain time periods of Rokugani history, although some artifacts may come in the form of "loot". But what items (besides "gaijin pepper") are explicitly or implicitly forbidden under the Emperor's edict? Various setting pieces I've seen have indicated fairly open trade of some more innocuous gaijin items. Are there any categories of items Rokugani simply never (officially) trade?
I think it is a misconception in the source material that Gaijin items is traded fairly openly, though I totally understand it can be interpreted that way since the books is a bit fuzzy on the subject.
After the White Stag Battle all gaijin items is completely forbidden. No matter what the item might be it is considered contraband and is illegal to trade and probably to posses too (though don't think the books says that outright). Of course, before then there are no restrictions.
However, except for gaijin pepper the enforcement of said laws might differ depending on the period and where in the Empire the trade takes place.
After all, those clans that do trade with it are either really secretive about it (Unicorn, Dragon) or are evasive about how the item where acquired (Unicorn) or try to justify that it isn't really gaijin they are trading with (Phoenix) or does it as outright smuggling (Mantis, Tortoise).
For example, the unicorn sometimes sells gaijin items openly (mainly curiosities and art), but when they do so they justify their existence as being inherited from his ancestors, or as being discovered in the dusty bottom of a warehouse that has not been opened in centuries.
So all trade with gaijin items happens under the table so to speak. However, the illicit trade seems to be fairly brisk and again with the exception of gaijin pepper doesn't seem that have any restrictions though most items I've seen as given examples have concerned luxury items, often of a smaller nature. Pg 252 in Emerald Empire gives a good example on how the tortoise clan deals with the trade.
(Thanks to MACN)