Suppression devices for paintball markers and air rifles are legal and do not require sort of federal licensing, $200 tax stamp, or permission from your local CLEO. Provided that they meet the criteria in the most recent ATF ruling, there is no problem transporting them either.
Almost all of my Sheridan markers have "paintball mufflers", which is what I call them so they are differentiated in everyones mind from "firearm silencers" which they absolutely are not..
This is the most recent ATF commentary on the issue:
If customers send you their markers, and you permanently attach a barrel, you may then make a "paintball muffler" provided that the porting of the markers barrel is integral to the functionality of the device. When your customer receives the marker they would need to get permission from the ATF and local CLEO and acquire a tax stamp if they intended to remove the barrel (which is considered "building" a firearm silencer), though the can itself can be removable for maintenance purposes provided it does not function on its own.
With the proper disclaimer and a small weld it should be easy to put these on autococker threaded markers. In the case of markers with an integrated barrel/breech (Automag, Tiberius, TiPX, etc) it would not require a weld.