I'm writing this for the non paying buyers, also known as NPB's. I've had a couple and know that its not uncommon for this to happen sometimes in this business. Murphy's laws says that some things can or will happen. A mistake on bidding, and emergency that is unavoidable, or sometimes you change your minds. If this does happen, you should make an effort to contact the seller to let them know that the transaction will not be completed.
Most sellers are a pretty understanding lot. But when a buyer just lets it go and doesn't pay, it may cause the NPB negative feedback to come to surface, which neither the seller or the buyer really wants. If a seller is informed, we are more likely to work with the buyer to leave feedback showing that we accepted the buyers decision and that we appreciate the honesty and for letting us know. This also gives the seller the opportunity to put the listing back up quickly. Especially if there was a person that wanted that item, but lost to the NPB.
So if you are a buyer, new or a veteran, and you cannot finish your business dealings, don't wait for the Grim Reaper of the feedback. Your reputation as a buyer could be put in jeopardy. Contact the seller and see what can be worked out.