I buy a lot of stuff on e-bay, have been for about 3 years now. I've never had a serious problem. But I did have a guy who didn't ship my item for about two weeks, when I filed a complaint with pay pal, he got his act together. I would definitely recommend to use pay pal. And don't bid if they don't accept pay pal.
All of the advice in previous posts is good, and here are some other things I do:
I only buy from sellers with at least 50 feedbacks and 100% postitive, I make sure I know the new cost of what I am buying and will not buy anything on e-bay for more than 75% of the new cost, including shipping. I also pay attention to every detail of the listing before I bid, especially all pictures. I don't buy from sellers when it is obvious from the grammar in their product descriptions that English is not their primary lanquage. Call me predjudice if you want, but its worked for me so far. And lastly, I try to buy from sellers I have had past success with.
For example, Hobbe-mart is a seller that I like to buy planes and parts from because they have a very good reputation, give very accurate descriptions, and are quick to resolve any problems that might occur. When you buy from individuals with few feed backs or less than 100% positive, you are taking a little more risk, so pay attention.[8D]