> My problem here is how someone knows they are diabetic.

Again, I don't really see why this would be the problem of the person selling it, unless you prefer living in a nanny state.

There are a multitude of OTC and other readily sellable stuff that if misused can cause death. Complaining about selling insulin freely and the dangers of it in a place where guns are sold like candy is a farce.

Make them sign a waiver and be done with it.