Yeah I hate that as well. I have used Paypal buying and selling on EBay for a year or so and I am far from being a big customer and I have followed the rules both on EBay and PayPal and never had a problem, as some of the folks here.
I have found that communication is the key to successful dealings on EBay when both buying and selling. If you do not respond to emails or delay shipping, even for good reason and do not let the buyer know, well then you are asking for trouble. If I am waiting for payment through M/O or other I always advise the customer when it is received and when I will be shipping. The same applies if I am buying, I advise the vendor when the payment has left and to please advise me when they receive it.
As I said never a problem yet.
Oh yeah I have been an EBay member since 1999 Only 1 negative feedback and that guy was an idiot anyway