It only makes sense to sell to whoever comes up with the money first, but once a deposit is accepted, it's a legal contract. Don't promise to "Hold" a car for anyone unless they give you a worthwhile deposit. I know it sounds raw, but I have been burnt many times by good intentioned people who can't follow through with the deal for various legitimate reasons. Sometimes things don't work out as we think they will. Sometimes it's hard because a prospective buyer will be travelling quite a distance, and doesn't want to make the trip for nothing. I won't agree to hold the vehicle, but once we agree on a date, I will agree not to show it to anyone else on that date, so the prospective buyer can be at ease about not haviing it sold from underneath him while he is in transit. If it sells before that date, I can let him know. If he doesn't show up, I've only lost one day, and haven't turned away a possible sale to someone else.
Personally, if I could sell an old Caddy I have, I be over in a heartbeat with the cash.