Another vote for the cash only option. You could very well do fine with PayPal, cashier’s check, or whatever, but if you want to be as close to 100% sure as you can then always insist on cash.
And don’t let buyers sway you with their particular situations or get you to accommodate their schedules. Your policy should be “first person to put $xxxx in my hand gets the bike.”
In the grand scheme of things, riding a motorcycle without a license is pretty low on the list of crimes. You have to pretty much be an idiotic moron to even get the police interested in you while test driving a motorcycle, let alone get pulled over, and if pulled over, not be able to convince the cop to follow you to the guy who has your license.
And, if you are an idiotic moron, I’d just as soon you take offense at my keeping your license and leave than ride my motorcycle.
To be honest, however, I’ve test rode a few motorcycles without leaving anything other than either the motorcycle or car I drove to the guy’s house. If asked for payment to let me test drive it, I would have left. I would never take the cash to buy a motorcycle on my first look at the bike. I pretty much assume the seller has the job of convincing me to by the bike, not me to convince him I am worthy enough to buy it. If the seller wanted to hold my license, I wouldn’t have a problem, though.