It's the nature of the beast. Sometimes you will get bad ratings, negative feedback, rightfully or not...however there are also many positives of eBay which obviously is not the focus of this thread.
There are ways of ensuring or maximizing your earning potential (and filtering out the problematic bidders who might complain) through good, long, and detailed item description, lots of high-res pictures, and pretty much making sure that your item stands out from all the rest. It's not easy, but it does help manipulating the bidders' psyche and swinging them onto your item(s), instigating ferocious bidding wars, etc., especially when it comes to rare amiga hardware. :-)
