You can't .
The seller can.
Contact the seller -- there is a link to him on the listing-- and politely explain your problem. The reason to choose is 'other".
Ask for a Mutual Cancellation.
When it arrives (and the seller can refuse, so again, polite) agree immediately.
The transaction is voided, your seller gets his fees back (which can be 20% of the selling price, by the way) and both of you can leave feedback.
Unless the seller was doing something horrible you should leave positive feedback. "Thank you for your prompt attention.' for example. Mentioning the cancellation may cause him problems in future.
