you need your receipt in order to prove there was a contract between you and the seller
Since you havent purchased this there is no contract or no seller for you to go to.
You only have the hardware warranty provided by the manufacturer. For this you don't need proof or purchase. The warranty is provided with the product, how you came about that product (eg being given it as a gift by a friend) makes no difference to their warranty process.
Theres no need to lie, just tell it how it is when you sell it.