The ads said that you need a Class III permit to buy them.

Last time I was casually browsing gun parts (I still have a ‘retro-M16’ semi-auto project I’m building – waiting for the proper upper receiver) I noticed people selling full-auto parts. You can’t even make a drop-in auto sear since (I think) the late-'80s, as it is considered a ‘machine gun’ in itself. A drop-in auto sear you can probably make with a hunk of metal and a file (plus a spring and a pin). IANA machinist, but machining the receiver doesn’t look very difficult. There are two ways around this: Machine the area inside of the lower receiver to match M-16 specs and drill a hole for the pin or make a drop-in auto sear.

The M-16 automatic sear will not fit in an AR-15. You also need an automatic sear, or else your hammer will follow the bolt carrier and you may detonate the cartridge before the bolt is locked. These are the hammer, trigger, disconnecter, selector, and bolt carrier. They directly replace the semi-auto parts. The AR-15 is very easy to convert if you have access to M-16 parts. Dunno for the MAC-10, but many such as the AR-15 are definitely manufactured now so you’d need a whole replacement receiver.

Mac 10 semi auto sale