Make really sure that you are not bidding more than you are willing to pay or can afford. Perform one more sanity check. Know thy bids I don't trust those automated notices from eBay, and my advice is not to rely on them. Keep track of your auctions yourself, either using my eBay or perhaps your own system. I use an Excel spreadsheet as a database for all of my stuff.
All bids are serious.
Some new people are tempted to bid on many auctions at once, then back out of all but the best deal, whether retracting or the NPB express. I advise against this. You will get lots of negative feedback, and eventually kicked off eBay. When bidding, assume that you will win every auction that you bid on, and that you will owe your True Max Bid. This is unlikely, but working from this assumption will help prevent your bank account from going bust.