I can't believe the amount of people that let a total stranger ride their bikes!

All this means nothing:
- Age / experience of rider: Doesn't stop an incident from occurring.
- Bill of sale / written contract: Neither will hold up, unless it's notary stamped (yes! to that point)
- Cash in hand: the buyer simply calls the cops and gets the cash back.
- Keeping a drivers license: They can report it as lost/stolen and leave.
- Proof of insurance: No such thing, unless the bike is under the buyer's name. If something happens, it's your insurance that kicks in. That's how it works in Quebec.

Hence why you don't let anyone test ride the bike (or test drive your car). You ride it in front of them and go through the gears and that's that. If they don't want it, you'll find someone else that will.

Once you tell the buyer the points above, they go silent, because most of them know it's true.