It's perfectly legal to buy, make or posses an airgun silencer. It must be made specifically to fit an airgun and only intended for that purpose.

According to the ATF, any airgun silencer that's removable, not an integral part of an airgun, or is removed from one in those two categories, could be considered an illegal silencer. Intending, attempting or actually mounting one on a firearm is illegal. If you're not planning to use one on a firearm, you're not breaking the law.

To put this in a clearer context, if you're in possession of an automotive oil filter, you could be arrested and prosecuted for possession of an illegal silencer. There would have to be evidence you intended to attach it to a firearm. Oil filters are widely known to make very effective silencers. Any object that can reduce the report of a firearm could be considered illegal if used for that purpose. You aren't going to be arrested or charged with a crime for simply possessing an airgun silencer. There has to be reasonable evidence you mounted it on a firearm or intended to do so.

There have been cases where this happened. In those cases the offenders were under investigation for felonious criminal activity. They were also in possession of, or attempting to acquire, a firearm and airgun silencer.

 If you're not planning to mount an airgun silencer on a firearm then you''re not breaking the law. Just don't do something stupid like carry or store a firearm with an airgun silencer or oil filter. It might also be a bad idea to buy an airgun silencer if you don't own an airgun but do own firearms. These actions could look suspicious to law enforcement. Even then, they would need evidence of illegal intent to arrest or charge you.