Because of the way agencies do their reporting, there is good reason to think that some considerable share of the weapons reported as “ghost guns” are not homemade firearms at all but rather ordinary firearms that have had the serial numbers removed.
This raises the question: Why would a criminal who has acquired a firearm illegally (or by any means other than a retail sale) bother to remove the serial number, when there is no way to use that number to track the firearm back to him? My first guess would be that these guns come from large, organized straw-buying operations, which remove serial numbers in order to keep guns used in crimes from being easily tracked to the proxy buyers they use.
In other cases, the proxy buyer may be someone close to the criminal (sibling, grandmother, girlfriend, etc.) and the serial number is removed for that reason.
In any case, straw buying is a much larger problem than is almost anything else gun-control advocates talk about. It is a shame — and a scandal — that these cases are so rarely prosecuted outside of the context of major organized-crime investigations.