You don't need people you need robots.
Seriously.
You have about an hour to cancel a mistaken purchase, and it is done by clicking a button. Phoning means you missed the deadline.
You now have two options, but they start the same.
Contact the seller. There is a Contact Seller link on the listing.
Politely explain why you want to end the transaction and ask for a Mutual Cancellation.
IF the seller decides to grant the request, agree immediately when you get the email and leave positive feedback. Something like. "Thank you for all your help" because sellers don't want to make it obvious that they will cancel transactions. Some would take advantage.
Or the seller may decide to open an Unpaid Item Dispute. She can do this between 48 and 96 hours after your purchase, depending on her location.
With a UID, if you do not pay, you get a Strike on your buying record which will make it more difficult for you to bid in future. You will not be able to leave any feedback.
Any time you have a problem with a transaction, your first action should always be to Contact the Seller. Always. She has more options than an underpaid clerk in a call centre in Utah.