You have to be very careful with buying from ebay, personally I wouldn't buy anything for a large sum of money from ebay, if so make sure that you use an approved payment method, such as paypal, check the ratings and feedback (they can be forged) and if you can use a credit card, at least that way you have a method of getting your money back if it is a scam.
It is definately a risky buisness I have used it to purchase small items, but I don't think I would spend anything over about £75 on goods brought from there, I have had friends who have lost money, and been scammed from them, but nothing bad has happened from me. For computers, its definately risky, but that is my opinion and I think there are far better ways of getting a good deal, computer fairs, recognised traders etc....
Proceed with caution!!!!