If it was a power seller, they normally run themselves as a business nowadays, if you kept the item description etc. it may be possible to pursue through the small claims courts. The fee depends on the size of the claim, the smallest being £50 I think and can all be done online.
As for paypal, I always try to get cash on collection on large items, as Paypal fee's are extortionate for small time sellers. I got clobbered for £12 fees selling my Plasma for £360, this is after other Ebay fee's.
Ebay is fast becoming a shop only zone. Almost no auctions anymore and just buy it now's all of the same price. Harder and harder to find a bargain.
To Fox: Have you tried FAB for your BMW parts, they are very good and have very good stock levels. BMW are an odd company, many engine parts are a few £££'s but you ask for a bit of rubber trim or something and they want 100's?