I was thinking he removed it after purchase, but then I remembered, this is Florida. They don't give a shit about ghost guns. They simply aren't restricted.
Well the whole "ghost gun" thing is poorly communicated by legislators and gets mixed with a lot of different stuff.
First, there are fully 3d printed firearms or lower receivers (what is legally considered "the gun"). That's how the "ghost gun" thing started IIRC.
Then there are 80% lower builds. Kind of like building your own PC, except you can't legally transfer it ever without serialization.
Both of those are very much in the hobbyist realm and far too much a pain for someone wanting something usable "out of the box". There are better ways to illegally obtain firearms.
Then you have private sales, which is commonly referred to as the "gun show loophole". Not all states require background checks or affidavits for private sales, or sometimes only for pistols and not long arms (rifles, shotguns). Existing federal law does not cover private sales IIRC.
Then there are illegal sales/straw purchases, and that's where I'd hazard the bulk of illegal acquisitions come from - lying on paperwork (purchaser or dealer) to make the sale go through, and destroying the serialization at some point along the way.
Edit: my knowledge on the topic isn't perfect by any means, so someone more knowledgeable feel free to correct me (I think I'm roughly on point though)