Never heard of this and have bought and sold many.

Like from a dealer/pawn shop or rando?

Never heard of this

I sold some guitar equipment lately and had to thumb-print both times.

Different stores too.

Never heard that before.  Maybe it's just a pawn shop thing.

I know various municipalities have encouraged pawn shops to require fingerprints for more commonly stolen items, but I wasn't aware there was a law requiring it.  They often require a photo ID for items, as well, so the fingerprint (thumbprint) is extra.  KRS chapter 226 is pretty simple, and is silent on that.  Clearly firearms would require ID, and local law may require ID for things the state doesn't mention.  I think some pawn shops enter their intake into a system that is shared with law enforcement, as a supplement/replacement for their records and reports.

 They do.

I remember being on a ride-along with a Lexington officer and he ran my name on his Panasonic toughbook.  It pulled up a few items I'd sold to a shop a few weeks before.

This was probably 15 years ago.