Why are you supposed to be 18 to use ebay??? I am 15, had an account since Feb and have 23 positive feedback (100%). I don't see why you should have to be 18. I also have a paypal account although I'm 15.
People who offer goods for sale on eBay, or who make bids on them are entering into a legally-binding contract to either supply the goods or to make payment. Failure to honour the contract will definitely be a civil offence and will probably be a criminal offence as well.
UK law defines people under 18 as 'minors' which means that they're too young to enter into any legal contract which, in turn, means that they can't trade on eBay.