Access the Black Market:
When you get the 6th member of your party (Maverial) go to the Odd Headquarters and go down to the basement. Go over to the little robot and zap it with Maverial weapon. It will then let you have access to the Black Market.

255 of Any Item (all three main characters needed):
Before the battle use the arrange command. Put the item you want to duplicate at the end of the list. Rearrange the item screen. Get in a battle and have the first two characters use heal berries. Have the third character switch the heal berry where with the item you wanted to duplicate. Once you're out of the battle check the first item slot and you will see 255 of the item you wanted to duplicate plus the origanal(s)! If you sell all 255 duplicates they will still be there and you will get free money!

Thanks to Revolution reader Shawn Flynn and Mike Angel!