I don't understand why people do this. I've been trying to sell a 1974 BMW 2002 for a buddy of mine and all I get are tire kickers. They ask for loads of pictures to be emailed to them, say they're going to buy it, and then I never hear from them again. I have pocketed one non-refundable deposit so far.
When I buy things, I offer to send a deposit so the seller knows I'm serious. Even if it's just $50, it gets the deal sealed (and is something I can afford to lose if the seller flakes on me or the item turns out to be not as represented).