Delete everything from (new) mobiles sent back to the shop?

Every traveller relies on a phone, for phone numbers and as one of the latest ad says, so the satellite navigation can tell you where you are. What could go wrong? Your new, upgraded phone.
A woman is worried because she took a faulty phone back to the shop and the new owner can see all her photos and new emails and bank details and passwords being updated.
Others says she should have deleted everything and re-installed icloud with a new password.
But you don't delete everything from your new phone after spending two days loading up all your phone numbers. You take it into the shop saying 'there is a problem' hoping they can fix it. After half an hour they give up and call the manager.
He gives you a new phone and promises to wipe the old one - or says he'll get it fixed and give it back - he then turns to three people waiting impatiently behind you.
By then you are late and trying to check the new phone is working and rush off.
When you phone the shop next week they tell you to keep the new phone. They can't find where the old one has gone but think it went back to the manufacturer and got binned.