Here are a few simple guidelines to help new ebayers. These are founded on my own experience of ebay over the past five years or so, as a seller and as a buyer.
It is always best to treat other members with the respect that you would expect from them, even if sometimes that respect is not returned. Set high standards for yourself, I recommend patience and understanding when dealing with others and the following of the correct protocol in times of dispute. If you stay courteous and businesslike at all times then you will have nothing to regret at a later date. Bad feedback should be left - only as a very last resort when all else has failed. Even then it should be factual and civilised - simply to warn other buyers and sellers of an ebayers shortcomings.
Always remember when you place a bid to stay within what you will be willing to pay (I know this sounds obvious but many get tied up in emotional bidding frenzies and end up paying far more than they intended). Work it out early and place your top bid as late as possible to avoid being 'run up' by other ebayers who are often just playing with the other bidders or miffed to have been outbid. Winning at any cost is not a good policy, unless you have just won the lottery and don't give a damn about the amount you have to hand over at the close of auction.  Pay quickly when an auction ends - this makes life simpler for everyone - similarly dispatch quickly once paid if you are a seller.
Most of the things that make ebay trading simple are just common sense, good communications and respect for others.
Hope this is helpful to someone.