Contact the seller and tell them it was an accident. I would suggest not ignoring it if you want to avoid an unpaid item case on your account. A bid is a binding contract, some people even been taken to court for not paying. In most cases when people don't pay for items I sell I simply open a case on them and let eBay handle the rest. The end result is an unpaid item strike, if you get to many you could be blocked from bidding on eBay.