iPhone 4: Questions about selling it

[deckeda]

Gazelle and Nextworth and Swappa can give you an idea of the phone's value. Updating the OS is optional when syncing or backing up. The message I wouldn't worry about at this point. You can remove some thing during a sync by deselecting them/replacing with nothing etc and the phone has a Reset settings function look around. Don't fear iTunes, just read the dialogs you come across.