The 80% lowers that you are referring have no serial numbers because they are incomplete. All it takes is a drill press and a jig to make it complete. That’s a lot less expensive than a 3D printer. I am not sure if there are any states that would require a serial number to be stamped on the complete lower. (According to the ATF it isn’t a lower receiver until it is complete. Any time before that it is just a hunk of metal, hence no FFL transfer.) I would think this below the radar for most state representatives, they are just learning the difference between mags and clips.
The supply of the unfinished lower receivers has pretty well dried up from what I can find.
The question to be considered is the act of drilling the receiver blank illegal in the state you live in. It is my understanding that in most it is not.
The only way I would be interested in building an AR type rifle is in 308 caliber. I am not at all interested in a 223. In my opinion the 223 is too light a caliber for anything I might use it for.
A heavy bullet for 223 is about 70 grains, 308 is commonly loaded with 180 grain bullets. There is a huge difference in impact energy with the 308 180 grain load than the 223 with 70 grain.
I knew a guy about 15 years ago who finished a couple of 80% AR receivers. He then engraved a pic of Ren on one and Stimpy on the other for the S/Ns.