To start this out, go to any dungeon in the game. Bring all of your most expensive items, you can check this beforehand in any town. (early game you are going to have the best luck just unequipping all of your weapons items and armor and selling healing items, just be careful you do not leave the game idle after doing this as if you wait a long time i have had the glitch not work; do not worry as when i have done it actively without taking a break it has never failed me.)

Open up the item shop, tap the chest at the bottom and begin selling your items. I recommend using items that stack just because for the most part you can sell in stacks of ten which makes the process... ten times faster. I am not sure why the text is glitched but i am using phoenix down* for this, which i believe can only be found in the purchasable gold chests. (this can be further helped by using the duplication exploit, which i may make a separate guide for but you can find if you look up.)

once you are done selling everything you want to sell, open the map (the dungeon doors button).

From here, press the arrow in the corner and open the bestiary menu.

Once you see the bestiary menu, press the X button at the bottom to close it. You should see the file select screen with the dungeon in the background. Choose the save you are currently using. It will take you back to the main menu, and when you load the game you should see that you have the gold for the items you sold but also the items you sold.