I have always required cash in hand and a closed sale, but also usually offer the potential buyer up to 10 miles and/or 60 minutes test riding time (or mechanical assessment) to return the bike, no questions asked, as long as it is in the same shape and not wrecked, pulled apart, etc. For liability, I simply won't let buyers ride my bikes, but will give them a narrow return window so they can have some buyer's protection and peace of mind.