I buy loads from Ebay.
Really depends what you want to buy.
For me, I try to look for people who are local - so I can pick items up to avoid postage charges.
I also look at feedback (not having 100% won't put me off, but I will check up)
Watch items and bid at the end (use bidnapper or auction sniper if you really want an item)
Have an upper price limit and stick to it - it's easy to become emotionally involved (well, it is for me!)
Read the descriptions carefully and ask questions if you aren't fully sure of something. If the answer isn't clear, don't bid.
For me, I always stick to personal sellers, but again, this will depend on what you are looking to buy.
I love e-bay and have 770 + feedbacks, I've never had a really bad experience and only a very small number of not great experiences.