1. Always use PayPal, it's much safer than MO's or Checks. If seller doesn't send you the item you can flag both PayPal and eBay.
2. Always buy from sellers that have at least decent feedback, and mostly positive. (If negative make sure it's a stupid problem, not like he conned people).
3. Ask questions! If a seller replies fast and knows what he's talking about, that's always a plus.
4. If possible, get it shipped USPS Priority over First Class. First Class can lag for a couple weeks, USPS is 2-3 days and the extra money is well spent.
5. BID AT THE LAST MINUTE! When people start their bidding wars with 10 minutes to go, prices can go very very high, and often higher than if nobody paid attention to the auction and somebody sniped it at the last minute.
6. Check "Completed Items" to make sure you're getting a good deal on your item. (i.e. GeForce 3's go for ~$50-60 on eBay, don't pay $70 for a GF3 if auctions beforehand went for much less)