We've covered this before. This thread
is good reading on the subject.
That said, Brian's correct. These aren't illegal on the face. However, when one attaches the oil filter and uses it to suppress the report, he is manufacturing an NFA item. It doesn't matter whether or not we agree; it's the law.
The people selling these cannot be ignorant as to how they'll be used, nor can they be unaware of the potential consequences to their customers.
For this to be legal, you'd actually have to serialize and register the oil filter you plan on using. Just that one
. When it wears out, you'll have to serialize and register a new one. Racking up the $200 tax that many times outbalances the cost benefit of using cheap media.