People sell phone all the time while under contract. It's a general practice to sell it completely stock, that way the buyer can still claim the manufacturers warranty. Though it is often recommended to wipe the device before selling it so you don't leave any personal info or files behind.

Apps that you bought on the Google Play store will transfer between phones as long as you use the same Google account.

 

Sim unlocking it would potentially increase its value as well as the size of the market of who could buy it. Just make sure that it's AT&T that unlocks it otherwise it gets into illegal mess that sim unlocking has become.
Factory reset should clear things out, but flashing a factory image through Odin would absolutely wipe everything out.

 

It's more of an issue that the factory reset function isn't actually a reformat. It's command to the recovery partition that wipes certain directories while leaving others intact. Custom recovery tends to leave media such as pictures, music, etc untouched. Stock recovery might clear those out, but I'm not 100% positive since I haven't run factory stock on a device for very long time.

 

You are correct. Stock (3e) recovery will wipe everything on a factory reset, but only by deletion of the contents and/or directories. It does not actually flash over the contents like an Odin flash will tend to do.