Before addressing the actual claims made by various sellers and their authenticity, let’s talk about the Terms and Conditions of Sales (T&C’s). Does the shop you intend to buy from have written T&Cs. If not, how do you expect them to enforce their guarantee claim?
T&C’s should be dated (i.e Version 4; updated 18.08.18). Otherwise, the seller can change them as they please; and what was true at the time of purchase isn’t any more when you claim your warranty.
Most reputable manufacturers exclude from their warranty the filling and fabric used. The foam life-span is directly dependent to the frequency of use and the weight of the users. And the care given to the fabric is totally depends on the user's care. PU fabric, for instance should not be exposed to direct sunlight or heat. How can the manufacturer be sure that it wasn’t the case? It is impossible.
If a seller suggests that the foam and/or the fabric is guaranteed, that should raise some concern; it is most probably a false claim.
Despite strong quality control, factory defects do happen. But what differentiate sellers is the way they handle these issues. The best way to verify if a seller commits to its promises is to check online the reviews of customers.