as a pretty serious ebayer, i think it is brilliant for finding scarce parts at (occasionally) really low prices, eg NOS heat shields for my Z1000, totally NLA from kawasaki and sourced at 20 quid for pair from US, result!
you have to know whats available and what isnt though, and market is very fickle, i only got ten quid for a pair of original Z1 instrument covers, people pay 30 quid for chrome replacements all the time, that flabbergasted me.
re bikes i'd say unless you have something really collectible like a mint Z1, the bids are really poor much of the time and having had good results here, if you arent in a hurry, use this site first with some back up ads in free mags such as motorcycle mechanics.
finally, if your bike is truly mint, wherever you advertise it, dont sell it short, there is a serious amount of old nails out there!