A bid or commitment to buy on eBay is considered a contract and you're obligated to purchase the item. An offer is the same as a bid. When you made an offer it says that it is final.
The best bet is to contact the seller. Most sellers will be nice enough to cancel your order.
If the seller doesn't agree to the cancel the transaction and you don't pay for the item, you'll get an unpaid item be recorded on your account. If you get too many unpaid items recorded on your account, your account can be limited or suspended.