What if they wreck, get hurt, hurt someone, steal it? What is your insurance company going to tell you, especially if you say you rented the bike? would you still be covered? Is it possible to fake a credit card deposit, or use a stolen one?

Personally I couldn't do anything more than swapping bikes for 5 minutes while out on a ride with a friend.

I really don't see this idea working out well with all the liabilities

also, no matter how good of a mechanic you are, parts fail. You can't control it 100% of the time