I want to buy a used iPhone, and since im looking for a cheap price ill prolly go for one close to a 6S series.
Let me know what you think and what should i be looking out for? What's going on with the bending issue?
Check battery life.
Check for signs of drops or unusual gaps or tool marks.
Check it's not reported or stolen.
Check it's been wiped and isn't linked to an iCloud account (so it can't be remotely locked).
If it seems to good a price, walk away. These things have a known value. If it's significantly cheaper, there is usually a good reason.
Pay through PayPal or similar for some buyer protection.