I don't usually do it around here, cuz this forum is a much calmer B/S/T forum, but at other places, I will do the same type of thing.
I've had a lot of experiences with people who "call dibs" or express interest and spin their tires for days. I have turned people away b/c I had someone interested already, and in the end wound up with an unsold item.
Like I said, I don't usually mention it here cuz it's never come up, but everywhere else, my item isn't sold until I have the money. I refund people if I get multiple paypal payments and just apologize (its happened twice). Of course I usually have a note in my thread saying that this is my policy which cuts down on people's frustration.