some of it depends on the situation.. if they have a car there or another person, it's a lot less likely they will bolt.. like for instance i just sold a bike to a couple who drove 30 minutes from home to an unfamiliar city.. i didn't ask for any collateral when the guy rode the bike cause his car and his girlfriend weren't going anywhere

a potential problem with asking to hold onto the money is that they might not have it.. in this case the buyer went to the bank after they test rode it

it's kinda a touch and go thing.. gotta use your spidey senses