If the part really is a conversion that works on a real Glock pistol, then I'd guess that the part itself is the machine gun like a R/DIAS for an AR. 100% illegal to buy or possess, with or without owning a Glock, with or without intent to install it in a Glock, and probably with or without a SOT unless you have a post sample LE letter. It's completely different from silencer build kits because you can legally build a silencer if you have an approved Form 1, so there is a legal use for them. You can't build a machinegun legally.
There is an instagram account that is always advertising Glock auto sear conversion switches. I think they're 80% "paperweights" though that require very minor finishing to complete them, but he's straight up advertising them as "full auto for $125, just email me" - which I fully assume is a fed trap.
http://www.instagram.com/suppressorgunse/ The bit about Jeff Bezos being some kind of puppet master with a grand war on 2A RKBA in cahoots with BATFE sounds ridiculous, though. But nothing surprises me anymore either.