Be careful when trading in your iphone

My iphone was recently hacked. i have had to think very clearly as to how all my contacts were accessed and I have reason to suspect that this may well have happened when i traded in my old iphone at Mall of Africa. The entire process of transferring my information from old phone to new phone took an abnormally long time apparently due to internet constraints however looking back my phone was left completely accessible. i was re-assured that i was doing the right thing however the cleaning of my old phone was not done in front of me. I believe my contact list was accessed to be used at a later date. i would issue caution to anyone trading their old phone in.