I remember back when I joined SL over 4 years ago there used to be issues with selling items for 0 Lindens. Back then this is why demos were sold for the lowest possible amount... 1L. I think this started the trend that continues today. Being a merchant in SL, I guess I never thought about going to a 0 dollar amount. As others have said it's a great way to track in a quick fashion. I can tell you this much, the last thing we are trying to do is screw someone out of a linden...hehe.