My
Call the merchant and ask them the exact date/minute that they processed your transaction and what ip they have associated with the purchase.
Compare this information to what you were doing at that time, and your ip.
If you weren't online and the ip isn't yours, alert them to the fact that it's a fraudulent transaction, and you'll be charging back - maybe they'll refund and save you the trouble of paperwork. Call your bank, alert them to an impending credit or the fact that you're contesting the validity of a charge, cancel the card, and get issued another. Filing a police report (or "incident report") in these instances is never out of order, either.
If you were online, or if it's the same ip (or an ip in your area), ask yourself how drunk you actually were that night.
Either way, stop buying shit on Ebay with your credit card over the Starbucks wifi.
And Good luck.