best bike theft i saw was a girl dropped a book out of her purse or something. she stopped her bike, put the kickstand down, and walked back about 5 steps to get her book. as soon as she turned around, some guy walking jumped on her bike and rode off.

I was walking so i couldn't get him, anyone that saw it was kinda just dumbfounded.

but yeah, i only do test rides with the full sale amount in cash in my hand. If they don't like it after riding around the block i return the money, and if they're pleased we shake hands and walk off. hasn't been a problem yet.