how do you guarantee anything? You make a calculated decision and hope for the best. I've tried both variants of this scenario and the one that always works best is where a recommendation has been given. Generally speaking people running a business will try not to let down those who have vouched for their services because it affects future sales.
pretty simple really, he wants someone reliable, at a fair price and with a nice clean reliable car. You can't guarantee any of those things by 'sticking an arm out' on the street.