Concernes about S5 Rooted from Box

Hello all,

I recently purchased an S5 from eBay, at this time I won't post seller to protect them in event I'm just over worried.

It's advertised as new 'Comes with Original Box "not sealed"'

Everything seems great with the device however I've recently been "attacked" on a couple of accounts, eBay being one of them where someone changed the email address on the account. This came coincidentally (or maybe not so) a few hours after installing the eBay App.

And just today another website (Wonga) sent me an SMS saying my account may have been compromised. I've not used that service on the new device or installed any app. However checking today and it seems chrome's saved passwords from my Google account.

I'd also sold a Microsoft 950XL the same day I bought this phone, that was all factory reset and sent with a brand new memory card and I kept the card I'd be using in that phone. I looked at the eBay customer who'd purchased my phone to see if anything suspect may be going on there. I was half expecting to see dozens of phones purchased over a short period of time. Nothing there to raise any eyebrows.

So back to my point:

I installed AVG and the only thing it reported about the device is it had been rooted. This seemed very odd on a device that's new boxed and "opened for testing purposes".

I've noticed nothing out of ordinary about data usage, the phone's battery life seems about normal for such a device and the way I use it but have a little niggle about the fact it's been rooted and maybe there's something on there sending information without my knowledge.

AVG's ONLY report shows thus:

"Unsecure privilege mode detected. Your device is running in High Privilege mode (i.e., it has been rooted). We recommend using only genuine firmware. For more info, please see out FAQ"

Any advice/help would be greatly appreciated.