Bent my iPhone 6 Plus, what can i do about this? It was purchased second hand
Apple don't require proof of purchase for warranty service if the product went on sale less than one year ago.
Your best bet is to make an appointment at the Genius Bar and claim that the product is defective as it shouldn't bend.
The massive press attention on this issue is likely to make them more sympathetic.

(I used to work as a Genius in an Apple Store)