Despite all appearances to the contrary, shopping on Ebay is very similar to all smart purchasing. I buy most of our day to day items here now. My best tips are to write down what you need (not want!) in advance; compare prices online and instore, watch a few auctions of similar goods. Have a budget. It's easy to get carried away so remember there will always be another item!!!
It is easier to get bargains from the categories with lots of sellers - eg sheets, kitchenware, clothes. Buy goods out of season (eg flannelette sheets in February, swimming pool chemicals in June.).
Never bid with your kids - they encourage you to spend way too much.