While you are supposed to fill out a customs form (and I'm surprised the airline didn't give you one onboard), I don't know if mine have been collected more then 2 or 3 times total. In fact, on one trip where I had something I needed to declare and have it documented, they first refused the form until I insisted and then they brought me into the office.
Anything for personal use (already used) is generally fine. I wouldn't want to enter with a few new iPhones in original packaging unless I planned a visit to the local airport taxman.