I only sell large ticket items via this process because of the chance of flakiness. Generally, though, most transactions have worked out for me, so I just take the goof balls in stride.
A) I haven't had much of this happen, but it's probably just part of the process. Keep all inquiries as a back up and keep the item posted (ie don't remove your ad until the money has changed hands.)
B) Say that the price is firm (no haggling) in the advertisement if you don't want to haggle. Otherwise, everyone will want to haggle once they see the item and you can't really hold it against them. I always just expect to lower my selling price a little bit and when I am a buyer most people give me a few bucks off as well.
C) Tell these folks that you are sorry, but you can't help them again. Unless you think their reason is really, really good, or it is particularly difficult to unload item. I've had sellers want to meet at a neutral location and then be more than 30 minutes late. Uber-annoying!