Always check feedback. If there is any negative, click that to get a list of the negative ones and see what actually went wrong. Sometimes it's just tossers complaining about something that wasn't the seller's fault.
Just because it's on eBay, doesn't mean it's the cheapest. Check things like Google Shopping search to make sure you can't get a better deal elsewhere.
Ask questions. Even if they're pretty pointless questions. The main purpose is to make sure that they respond and are friendly and informative. If they're not or don't respond at all then avoid.
Remember that the onus is on the buyer to provide feedback first.
As stated, ask for photos of the actual items if they're not provided.
If you're selling, use Turbolister. But don't sell games on eBay as they're royally screwing sellers in that category right now. But that does mean, as a buyer you'll be able to find cheaper games than before.

A CPU. It sounds dumb now, but at the time I had no idea. They're pretty hot on people doing it now.

Just one tip: never buy a Michael Jackson toast.

Woo...just get my first eBay item: Legend of Zelda - Phantom Hourglass for AU$18. Not bad, though there are better deals where I was outbidded in the last few seconds by mere cents. I've developed some sense to spot counterfeit games, too (if possible, always stick to OFLC-rated games). Now I can see why someone could get addicted to this... :o

There is a certain addictive & competitive quality to it. A mate was here bidding on a PDA phone a few years back and the tension in the last few minutes was like the last few minutes of the cricket when its really close