I bought a second hand 5C 16GB in white off eBay for a pretty decent price from a private seller. No receipt was included in the sale as the seller bought it from his network. Everything has been fine and I've had no issues until the last couple of days. The home button has become very unresponsive - if I'm in an app and want to return to the home screen it takes sometimes 3 or 4 hard presses/clicks to do it. Getting to the multi-tasking screen (double-click) is even harder and sometimes takes 5-10 double clicks. It feels a lot different to my 5 which worked fine every time. Obviously I don't have a receipt and the phone has 11 months Apple warranty left. Can I just walk into/get a genius bar appointment at any Apple store and get a warranty exchange there and then with no receipt?