Going from the listing, the phone just needs unlocking, not unblocking. A phone being blocked is totally different to a phone being locked xxxjase. Locked just means that it's locked to a network and needs an unlocking code, provided by your network for a ridiculous fee or being taken to a high street store or a market for between £5 and £10. Blocking means the phone's imei number is blacklisted and the phone cannot be used unless the imei is changed, which is illegal. Selling blocked phones isn't illegal though AFAIK so eBay can't do sod all unless the listing is misleading and says that the phone is working when it isnt. There's a complaints procedure that you can file through eBay and through paypal.
The seller has all positive feedback so there's no reason to distrust him, just e-mail him asking him again if it is just locked to 02 and not actually unblocked. Try another 02 sim in the phone as welll and see if it works if you have one. If it doesnt work then the phone is most likely blocked and report the user to eBay and get your money back.
regards
--marty