Don't get caught up in the bidding frenzy. Set yourself a budget for each item you wish to purchase and don't forget to factor in the postage costs.
I beleive buying on ebay is one of the best ways of shopping but you must be careful that you do not pay more than the item is actually worth. The easiest way to do this is to set yourself a limit. Do some research, find out what the item is worth and then ask yourself what is the maximum I am prepared to pay for this item including the postage cost.
I have at many times starting bidding on an item only to find one other buyer keeps outbidding me and I end up just wanting to win the item no matter what. Is it the adrenalin or is it the will to win? I then find myself buying an item I didn't really want to pay that much for. For me, the best way to overcome this is set a budget. At the time when you start bidding the price is so cheap you just keep bidding but make sure you write down the maximum price you are prepared to pay and for your own sake don't go over it. There are many auctions on ebay which sell similar products. Check them out in your search and watch them in "my ebay". This helps you get a rough idea of what people have been paying for an item.
If you go over your budget you won't have the money to buy the next item you wish to purchase. Goodluck and happy bidding!