Strange Device and Computer

I noticed a strange device on my iTunes, so I removed it and changed my password. I can't deauthorise the strange computer that came with the strange device though as I've too recently "Deauthorized All" and can't deauthorise all until April 2016. I recently sold a factory reset phone, but clearly something didn't go right so I'm guessing that's where the device came from and whoever bought it has access. I don't want to phone Apple as I'm not covered by any sort of guarantee or warranty. Is there anything else I can do to ensure someone isn't accessing my account?