There are thousands of guns listed in the FRT. Many are transferred every day.
Are you referring to firearms that have appeared with a prohibited classification since the OIC was proclaimed?
Of course a sting operation could be set up to create a test case to establish the legitimacy of these prohibited classifications.
If this were to happen, I doubt it would be an officer operating independently - it would be organized and approved at a high level.
A decision would have been made to test the validity of a prohibition, and establish a legal precedent all the way to the SCoC if necessary.