Is it illegal to purchase the sponsored silencers/suppressors disguised as fuel filters on Facebook?

You see these fuel filters which are obviously suppressor/silencer kits advertised on your Facebook news feeds constantly. These people are obviously pulling the wool over Facebook’s eyes or they choose to ignore it because they desperately need advertisers. Is it illegal to buy and sell these kits? Or as like most things, are they so ignorant that they believe these things are fuel filters?