Here's the scam. If you are a FFL and have a SOT making you a class 3 dealer, you get your friendly Cops to write you a demo letter for one. Sometimes this requires buying an extra whatever and giving it to the department. Othertimes a charitable cash donation, or beers or just being buddies will get you one of the prized letters. Often times SOT's get friendly with the local sherriff for such purposes. All you need is a dumb letter. Less than a full paragraph really.
Then you fax copy of letter to Hk and buy one. A complete gun is in the $1500 range. When it arrives, strip all goodies from the lower and cut lower in half. Throw Lower away and write ATF doccumenting the destruction. Or keep it. What's left over is referred to as a parts kit.
That's what's being sold online for $4500. It cost the original buyer $1500ish. The rest is profit.
Just stuff to keep in mind. That's how the scam works.