Last month I bought a barely used iPhone 5s from a local seller with the intentions of giving it to my wife as an upgrade to her 4. Long story short, I ended up buying her a new 6 for Christmas and am keeping the 5s for myself. The seller made sure it wasn't attached to anybody's icloud, so you should be fine. I got a new sim card from T-Mobile, activated the phone, restored everything from my old 4s backup, and have been using it for a few days now with no issues. It's showing up in iTunes and I can backup, install new apps, etc. with no issues. It's just like brand new and all's well. In fact, it's under warranty until July of 2015.

So for the hell of it, I figured I'd prep my old 4s for resale. I went to the Apple site, entered my Apple ID, disassociated my 4s from my ID and thought I might as well add my new 5s to my ID while I was there. I entered the serial number and..."According to our records, this serial number is associated to another Apple ID.".

I contacted the seller and she was very apologetic and was also a little confused. This is a company who bought a few of these with the thought that they might switch to T-Mobile, but ultimately decided against it, which is why they were selling these so soon after buying. She's going to try to hunt down the previous user to see if they added it to an account.

As I said, I was able to set it up, restore from my 4s backup, everything is running fine. I've even downloaded and installed a new app on the thing without issue. So I guess my bottom line question is should I be at all concerned?

I think that just means that the original owner registered the phone for warranty purposes, when they bought it.
IIRC, I ran into that when I tried to add my iPhone 4 to my list of devices at Apple, under my ID. I had bought the phone used from a guy who had upgraded. It didn't cause any problems, for me.

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Roswell, GA (Atlanta suburb)

What rgG said. Associated with another account when registered with Apple.

But it wouldn't hurt to test Find My iPhone to make sure it works as expected.

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Saint Cloud, Minnesota, where the weather is wonderful even when it isn't.

I had the same issue with my 5S. Like rgG said, someone just registered the product to their account. I just didn't like it b/c I couldn't add it to my growing list of Mac stuff in my account.

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Thanks. And that's the thing. I'd like to be able to add it my Apple ID account, but as long as there's nothing negative other than it not showing up in that list, I guess I'm okay. I'm assuming I could also take advantage of the balance of the warranty, if need be. You'd think that if Apple acknowledges me as the new owner, they'd automatically disassociate the phone from a previous account. Oh well...

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wurm
Thanks. And that's the thing. I'd like to be able to add it my Apple ID account, but as long as there's nothing negative other than it not showing up in that list, I guess I'm okay. I'm assuming I could also take advantage of the balance of the warranty, if need be. You'd think that if Apple acknowledges me as the new owner, they'd automatically disassociate the phone from a previous account. Oh well...

Yep, the warranty follows the device, not the owner.
I had to take my mother-in-laws iMac in once for repair, why was that my job you might ask , and all they cared about was if the machine was under warranty, not who the owner was.
I think I also remember someone here buying a Mac off CL and then taking it in for some sort of recall repair, with no issue. My iPhone 4 also had some warranty left, when I bought it used. I didn't have to use the warranty for anything, but I was given to understand that it would not have been a problem, if I had needed to.

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Roswell, GA (Atlanta suburb)

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Speedy
But it wouldn't hurt to test Find My iPhone to make sure it works as expected.

I just had a moment of panic. I went to Apple's site to check the Activation Lock and it shows Activation Lock: ON. Then I realized that it was me who set up Find My Phone when I activated the phone. D'oh!