A bid or Buy It Now purchase on eBay is considered a contract and you are obligated to purchase the item.
However, mistakes sometimes happen, and many sellers are willing to work with you if you communicate with them openly and honestly. If you have not done so already, you may want to request the seller's contact information and give the seller a call. Let the seller know your situation, and see if you can work things out to everyone's satisfaction.
Request a seller's contact information.