Are you bidding on an auction ?
You can request that your bid be cancelled, or you can retract it yourself.
Did you purchase an item which you have paid for?
The seller, if she is like most sellers, will try to get it in the mail as soon as possible. If it is in the mail, she can't get it back.
The purchase will have to be delivered and then returned to the seller, before she is required to refund you.
Note that she paid fees to eBay and to Paypal when you purchased and those will not be refunded to her.
Note that this is a Buyer Remorse situation and she is allowed to make a partial refund.
While you can refuse the shipment, it will then be considered as Undeliverable. EBay does not require the seller to refund anything on an Undeliverable shipment.
If you open a Dispute instead, you will be required to pay for tracked return shipping.
Basically, it would be cheaper and easier just to return the other locally purchased item, or use that one as a present for someone else, , or give your friend two presents, or keep the eBay purchase for your friend's birthday.
Did you purchase an item which you have not paid for?
The seller can cancel the purchase, but she is equally able to open an Unpaid Item Dispute. When you still don't pay, she gets the fees mentioned above returned, you get a Strike on your Buying account which make it more difficult to bid or buy on eBay in future.
