A bid or Buy It Now purchase on eBay is considered a contract and you are obligated to purchase the item.
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.
If the seller does not respond in the way you hope, keep in mind that failing to purchase an item you have won may result in an Unpaid Item strike being filed against you. Remember, if you get too many strikes in too short a time period, your account will be suspended indefinitely. All negative and neutral Feedback you left for other members will also be removed permanently. In some cases, limits may be placed on your account in advance of suspension.
Request a seller's contact information.