I have no idea where you are getting this idea of "rental contract" from,
From the fact that a) the customer no longer has the product and the original retailer/owner does and b) the customer also has paid money for a product that they no longer have. Surely that is the very definition of a rental? It's certainly not a purchase is it?
basically within 6 months, any fault is assumed to have been there all along and finding any proof to the contrary is placed on the retailer. After 6 months and before 6 years (in the UK) this is not the case and the retailer is under no obligation to prove the fault was not there, you also must take into account how long a product is expected to last (A pair of trainers for example are unlikely to last 5 years with regular wear).
If the product is within the 6 years and within its expected life time, the retailer should provide repairs, a replacement OR a partial refund, taking into account usage.
The retailer is under no obligation to provide a full refund if the product has been used for longer than 6 months without fault when the buyer can not prove the fault was present on purchase.
So it is actual law then? This is why I am asking, I'm not being awkward or anything but am very curious as to whether it has a legal standing or is it yet another piece of 'retailer law' that they'd all like you to think is real but isn't.