If you have been ripped off when paying by Postal Order IMMEDIATELY call 01246 542091.
This is the number for the post office's postal order dep't.
Even though you have paid for the postal order it may not have been cashed by the recipient, and the post office can put a stop on it just like a bank can with a cheque.
This may be your chance to get your money back.
In future I will be paying by Paypal, because you can not rely on EBay's feedback system (if lots of negatives are left after a scam or fraud, and the seller says their account was hijacked EBay remove all of the negatives and restore their 100% record, which leaves no warning for future victims).
Hope this information is helpful.