If you want something specific from a merchant just quick-save before talking to him and if he doesn't have what you are looking for just reload and speak to him again because his inventory will be changed.
If the game is too hard in some situation, or if you just want anything to be easier, then press A
and export your character and start the game from begining by importing your character, you wont have anything from your inventory but you'll have the character level, skills,etc. you had in that part of the game
If you quick save before you open a chest, you can reload if you didn't get what you wanted and the items in the chest change most of the time. this works on 1 player, I haven't tried it on Multi yet
Did you know that everytime you open a box the item you will get is random?
When you need a particular weapon or other item find a box and quick-save your game before opening it.Then open.If you don't need that item just load your quick save and open it again.There will be a different thing inside.