The only way you can get a ghost gun is to buy the frame or action of the gun from someone who made it from scratch.
If you build a gun from parts and assemble it yourself, the frame or action of the gun, if purchased through a federally licensed gun dealer, will have a serial number on it.
It’s against federal law to sell a firearm with out a serial number as well as failing to record the sale of a firearm by a licensed dealer.
It is also unlawful to sell or buy a firearm with a altered or obliterated serial number.
There are exceptions for antique muzzle loading firearms that don’t use center fire cartridge ammunition.
Gun “kits” do not contain the receiver or action, which is the part that has the serial number on it and to which all the other parts attach to make a working firearm.
I believe this issue is just another loophole that might need to be addressed by legislation.
It is definitely not as much of a problem as some people would want you to believe it is.