Hahaha, that’s not a game issue at all. You must have selected trading with the Item menu, not the Weapon menu. FE3 has two separate inventories on each unit, one for weapons and one for items, as you can see. You need to select Weapon first, then Trade if you want to trade weapons.
oh my god 
I’m glad it was just a knowledge issue I was going crazy. thank you. I literally gaslit myself by accidentally trading successfully on a japanese non patched rom.