You don't.
If you are bidding, you can Retract your bid under limited circumstances.
See the Site Map .
If you do not fit the criteria for Retraction, you can contact the seller and ask to have your bid cancelled.
If you have won, you can Contact the seller and ask her to cancel the transaction. If you get a note from eBay asking if you agree to a Mutual Cancellation, agree immediately and leave positive feedback "Thank you for your help with this,' would be appropriate.
The alternative is not to pay, and losing an Unpaid Item Dispute. Those give you Strikes and many sellers automatically Block bidders with Strikes. Too many Strikes and eBay will close your account.
