damn that sucks.. the first time i let someone test ride a bike i didn't get any collateral and they had taken public transportation (so no car). They rode way up the block and i was like dude.. they could take off right now and I'd be screwed. luckily the bike was a total POS so it wouldn't have been a big loss.. and luckily the guy came back and didnt even want the bike because it was such a POS

getting a driver's license isn't even a sure thing since that could be a fake.. seems like the cash is the only fair collateral. also that way they can't lowball you cause you have their money