A bid or commitment to buy on eBay is considered a contract and you're obligated to purchase the item. However, if you feel that you have a legitimate reason for not buying the item, you can contact the seller and explain your situation.
Many sellers are willing to work with you if you communicate with them openly and honestly. If you haven't done so already, contact the seller through the listing in My Purchase history, explain the situation, and see if the seller is willing to cancel the transaction.
If the seller doesn't agree to the cancel the transaction and you don't pay for the item, an unpaid item may be recorded on your account. If you get too many unpaid items recorded on your account, your account may be limited or suspended.