Exactly what do you think a non transferable machine gun is worth in the US?
None of the guns you see cut up are legal for sale in the US. If they are for sale someplace else...then they are being sold there and these are really not impacting anyone being cut up.
If these guns were not cut up and resold as parts kits, they would sit in a heap outside someplace until such time as they were dust.
At least as a parts kit they can be used to breathe life into an exisiting transferable machine gun in need of parts.
What's that kit worth? With the mag about $50.