You could always add the ability to sell items at the shop for a portion of the value of course so it would stay balanced.
You could even make it so you can sell Red token items because of the rarity that you will actually get them from normal item boxes.

How about direct trading between players?

I thought about putting that into the suggestion to but then you would get people farming items on higher characters and trading them to their lowers.

This will cause a lot of problems. Supermechs allows multiaccounts. There will just be players that pass the tutorial, where they get some items, then just pass a few missions on each account to earn some money and sell all of their items to their main account to be forged to use their energy. There are some other ploys that can be done if selling items is allowed.

This has been brought up quite a lot in the past and many suggestions have been made (some more complex than others) to make this viable.

I'm sure the devs, if they see any of the suggestions viable, may implement this back in to the game. However it is very unlikely as the current system is working well right now and there is no real need for change (as weapons you don't need can be fused, and fusion needs to be encouraged as the top players normally have highly fused parts on their mech).