One way to avoid this is to schedule multiple people to come by at the same time. I've done this when selling a few of my motorcycles. That way you have a better chance of at least one person showing, and then if it's worth more to one of the buyers they can take it with them.
Seriously? I had somebody do that with me once. I showed them all the cash I had with me and then told them where to go. Seems like complete BS to me.