What happens once an iPhone can no longer power up?

My iPhone 5c suffered water damage and won't turn on at all. I want to sell my phone to a company that buys broken devices (uSell). My question is, will they be able to access my personal information (contacts, pictures, email, apps, etc) since I was never able to sign out of iCloud?

You can change your icloud password or use find my iphone to remotely wipe it, so if it is on and connected it will be wiped. I would just trade it in and upgrade to the latest because then you know the apple store or carrier won't do anything bad, and you won't get much anyway if water damaged.