According to this article https://www.guntrustlawyer.com/maryland-man-arrested-fuel-filter-silencer/ https://www.guntrustlawyer.com/maryland-man-arrested-fuel-f

According to this article https://www.guntrustlawyer.com/maryland-man-arrested-fuel-filter-silencer/ https://www.guntrustlawyer.com/maryland-man-arrested-fuel-fi... they have been sold on Amazon before. If they are considered a silencer by the ATF then it would fall under Amazon prohibition on selling silencers.