There are some well known companies who have eBay outlets for their return, refurbished and overstocked computer ranges. Usually they will offer a reasonable to good warranty with the usual consumer rights applicable. Buying from that source should be fairly safe.
If its from a private seller, then think 'buyer beware' and take all the necessary precautions and steps, because all sorts of good or bad could result if you do not.
As been already suggested, there are a number of £399.99 laptops available, some with interest free finance deals, which could save money and perhaps turn out as a bargain in the long term.