I'd be VERY wary of buying something as complex as a computer on Ebay. What happens if something doesn't work? How is their customer service and return policy? How do you know the seller won't vanish or go out of business in a week/month/etc?
Better off buying something from a known, reputable retailer than Ebay, IMHO. It's just not worth trying to save a few bucks to get crappy quality from an unknown vendor.