Lots of really bad information in this thread. But it isn't like the website is helping either.
They are not selling complete firearms. They are selling receivers that do not have a serial number. People prefer to do this because if they are creating a gun from scratch they will want to put "00001" or some other sort of serial number on it.
Now, to examine how somebody could commit a crime, or a mass murder with an "unserialized" firearm using a website like Ghostgun.com they would have to do the following.
1. Purchase a ghostgun receiver that has no serial.
2. Purchase a serialized firearm with the parts that make it a firearm.
3. Replace the parts from the serialized firearm and put it into the ghostgun receiver without a serial number... (this part is a crime) - this would be spending 500+ dollars to do the same thing a metal file would do....
4. Commit a crime or mass murder.
5. Ghostgun gets blamed for making criminals spend $500 on something a metal file can do. Gets sued, but hopefully makes a ton of money in the process by stealing $500+ dollars per weapon from unwitting criminals or mass murderers who we never properly introduced to a metal file.
edit: this would be the case for a glock.
AR-15s you can take an unmilled lower and mill the portions out of it to make an unregistered firearm while still being legal.
https://www.80-lower.com/blogs/80-lower-blog/how-to-build-your-own-ar-15-legally-and-unregistered