I eBay as a hobby only and have worked out my non-payers thus far (I haven't sold heaps of stuff in the 3-4 months I've been doing this) account for approximately 4% of the transactions I've had.

My full-time job is in trade and wholesale sales and our no show/drop off the face of the earth rate on picked/packed and purchased in to special order orders (often from some fairly major national companies) generally runs at around 12%.

On the basis of that comparison I reckon I do okay on eBay, as frustrating as a non payer is.