The single best way of buying PC components cheaply on ebay is this
Type in what you're looking for, tick the 'Search in item description' box also.
Change options to BUY IT NOW Only.... and NEWLY LISTED Items.
This is where the vast majority of bargains will be found, since sellers who are keen to shift their item's quickly will use a low buy it now price using a 3 or 5 day listing. Keep searching using this method and you'll save £100's over time.