Hi - just to add to the bandwidth of this discussion - if you are to sign out of your iTunes/iCloud/app store account on an iOS device it's possible (AFAIK) to sell the device and not compromise your account at all. The buyer will be able to use the device 'as is' or add new apps to the device with their own iTunes account. They will be asked for the original iTunes account password (and see the owners iTunes a/c email address) if they try to update any of the apps that had been left on the device - but it won't give them access to anything. They will lose everything if they want to sync with iTunes and add their own music/photos etc.
I always ensure that I sign out of my iTunes & App Store account on my iPhone as I'm pretty sure that it loves to keep checking with the Mothership when signed in, so it can let you know when any app has an update - this seems to use unnecessary battery power.