1. u can trade in the game because it allows you to have both players accept and etc and probably not alot of scammers..
2. u can find the codes on facebook all the time by liking "borderlands" fb page not borderlands2

they are always handing them out or people will post them here as i will do in a few mins

then u will go to the mainmenu and enter it under shift codes and use it in the chest and sanctuary giving u a gun/grenade/shield etc based on ur level so make sure your 30ish or higher unless u have more then one key
3. oh didnt read far but ya #2

4. over spending? when u start a new character you will ofc play through the whole thing and when u beat the game it will say you have unlocked true vault hunter mode which when you go to that you will have to do all the missions over again but it will b harder and you will get better weapons and loot and i havent deleted any of my chars i just make new ones
i have a lvl 47 assassins
lvl 50 siren
lvl 50 commando
lvl 30-40 mechromancer

lmk if u have any other questions