How often do bike sales occur with no text or email record of arranging to meet up?

I guess those arrangements could be made over a phone call only, in which case call logs could be subpoena'd, but that's not going to happen on the spot, and probably will never happen for a low-ish value item like most bikes.

And a thief could always use a burner phone.

But that's not a long-term scam, a thief could probably only get that past the police once. 2nd time hopefully they'd look at the registry and see a (contested) previous theft/recovery and inquire deeper.

But yes, it can't hurt for a buyer to request a written receipt. Or how about a selfie with the bike, and the buyer&seller shaking hands (that also gets a timestamp, and maybe a geotag as well)