When I sold my bikes I just had a written agreement for a test ride. It was a written "you damage it, you buy it" kind of deal while I held the cash in hand.

Personally, I wouldn't buy a bike under the 30 minute test ride provision because all they have to do is refuse to give you your money back until the allocated time or "go use the restroom" or the old "I'll be right back" and then you are stuck with it. Never underestimate the stupidity of people.