I quite offen buy and sell on Ebay - like everything it has it's pitfalls, the rules I usually stick by are:
1) Be careful buying from someone with no or low feedback, however everyone has to start somewhere, it's true that people can change their ID, but a little icon next to their username shows this, and ask yourself this question "Why did they change their ID?" - AVOID!
2) If something is bothering you about an item, ask the seller a question - if they reply that the item is complete and when it turns up on your doorstep incomplete, you've got a good case for complaining, oh and don't email a seller a question about an auction that is ending in a couple of hours!
3) Feedback can be a joke, there are alot of dealers on Ebay buying stock and reselling it a week later trying to make a quick buck! Take a little time to read through a couple of different feedback comments left by recent buyers and sellers, so you can build a good picture of the person you are buying from.
4) Study, study, study - check that other items aren't being sold for cheaper or at the same price, it is very rare to find only one of a particular item on Ebay. Plus what usually happens is that items come and go in trends, one time a vintage Scalextric Pit Lane can sell for around £80, a month later Ebay is littered with them and you can pick on up for £30 - just be patient.
5) If you really want something try an "Auction Stealer" (www.auctionstealer.com) its FREE and submits a bid for you in the last 10 SECONDS of an auction, a great tool. All you do is sign up, enter the auction number and the auction site it is on, set your highest bid and sit back and wait! And yes I know its cheating, but thats life!
6) Don't bother bidding to early on an item, it just pushes the price up. Wait to the last 5-10 minutes and then strike.
7) Look for auctions that have an unusual end time e.g 11:30pm or 8:00am, less opposition.
8) Try bidding in 'funny' numbers, e.g rather than bidding £15 for an item try £15.03 or £15.28 - most new Ebay users bid in round figures, that 3p difference could mean winning or losing.
9) Check postage, postage costs are the biggest area for cons, but do take into account things like packaging and the time it takes to post the item. However if someone is stating £6 for a single fly panoz (boxed) delivered via Royal Mail Special Delivery (which is next day delivery) and it doesn't turn up for over a week - that is a bloody con, oh can you guess this is not a fictional story.
Can't think of any more useful tips, but if I do I'll post them up.