My best tip is to Wait! People have things BUY IT NOW for two reasons, ONE is to sell the item quickly, and i know alot of people that only buy it now as they dont have time to search for the best buys. TWO is because the seller knows that if the item goes to auction, they are not going to get the price they want for it. This could mean a collectable item or even just a piece of clothing.
So if you have the patience and time, WAIT for your items, it usually means that you will get the item for a better price and save your self some money.
My other tip is when Paying for your item use paypal! It saves alot of hassle when you know that your money is protected some. If a seller does not send your item and just keeps your money, there really isnt much ebay can do about it, but paypal has the power to get your money back. And believe me people there are HEAPS of these sellers out there, that just want to rip other people off. Paypal also protects you if the item is not what you paid for. Make sure that you check the persons feedback and go with your gut feeling. If it seems to good to be true, then it usually is.
Ebay is really a good place to shop. You can get some pro loved jewellery and clothes on here cheaper than at a op shop, and save your self time and money buy not having to use the car or wander around a store looking for one item or driving 40 minutes to that speaciality store.