I am afraid that Ebay Uk is full of hacked accounts. I have sold an item worth £400 three times only for the buyer to be a fraudster using someone else's account and trying to get me to complete the sale outside ebay/paypal.
As a result I will no longer waste time selling via Ebay unless Ebay tightens up their bidding security. It would be very simple, forexample, for them to have a second level of password for bids over £100...
So to help you:
1) Only accept payments via ebay and paypal within ebay. IE ignore buyers who ask you to send your pyment details to a different email address.
2) Only send to Paypal verified addresses
3) Good luck contacting ebay - they hide behind their computers.