1.Check the seller.
2.Is the feedback score good? Any negative feedback - The new Ebay percentage system helps greatly.(run mouse over> 'How feedback is worked out')
3.Be carefull before bidding - read the details - is the item new? - Boxed? - Any damage (Not always mentioned in description - if not ask before bidding!)
4. Set yourself a limit and stick to it - Don't impulse buy - what is the item worth bought new in the shops? Don't know? Research first.
5. Paypal - Ebay use it for a reason to provide security for both parties - its saved me financialy on a couple of occasions!