You have a very valid point - it is a problem if the HDD fails and there is no installation CD.
Unfortunately, it's "not Microsoft's problem", and they are very unlikely to send you an installation CD.
They may sell you one, but at RRP for the product! You might be better trying to get and OEM one from the laptop Manfr., who might do so for goodwill.
But really it's down to the ebay seller to provide whatever came with the laptop. And even then, so many manfrs only provide a recovery partition on the HD that there is nothing for the ebay seller to pass on.
The lack of installation media from the vast majority of manfrs. is a shocking capitulation to MS bullying (in the name of anti-piracy, of course).
SD