I had a buyer once who didn't pay and didn't respond -- I sent them very polite emails -- "if there's a problem with being able to pay please let me know so we can see if something can be worked out", and after about five days I said that I would be contacting ebay about a non-payment -- and got a very nasty email back! "You can't even contact ebay until it's been 10 days, you can't leave negative feedback, but I sure can" -- they finally paid after a week, but didn't leave negative feedback. They're now on my list of non-allowed bidders.