If you're interested in making the max amount of money in the game (999,999), try the following. Just be sure that as you attempt this, you have at least one sellable gem or jewel in your inventory, like a Diamond. It must be unequipped. You must also have access to the Sword Brothers spell (via the Sword familiar).
Now, head to the Library, and go to the location of the Librarian, the game's only merchant. Stand next to his desk and cast Sword Brothers (Down, Down-Right, Right, Up-Right, Up (for two seconds), Down-Attack). If executed correctly, you can then talk to the Librarian as the spell is being cast.
Doing this will glitch the game, allowing you to access your inventory from the Librarian's own menu. Now, remember that unequipped jewel or gem we mentioned earlier? Equip it from this menu, then go to sell that very jewel or gem. However, it won't be sold. Instead, it will duplicate 255 times in your inventory, which you can then sell to the Librarian for massive amounts of gold, including the maximum amount of gold (if you have the patience).