The wealth by level assumptions already commoditize magic - that ship has sailed.
The current model assumes you will have certain wealth per level, and will have free access to magic items. So the character has a 3rd level spell that can change the magic sword to a magic Ogrehook. If the alternative is to sell the magic sword for half value, then have the wizard Craft an Ogrehook with the same enchantments, how different is that?
The wizard had to take a feat, maybe we have to pay full price for the mundane weapon (unless a character can Craft those) and it takes some time. The result is the same.
Add a restriction that the spell fails on artifacts, and set the king's sword as an artifact, based on its legendary history, and move on. Isn't the King's Sword, with that legend attached, worth way more gold to the right buyer than a "no history" weapon with the same enchantments anyway? So, wouldn't a valid purchaser be very willing to swap for a freshly enchanted Ogrehook with the same enchantments?
The current model assumes you will have certain wealth per level, and will have free access to magic items. So the character has a 3rd level spell that can change the magic sword to a magic Ogrehook. If the alternative is to sell the magic sword for half value, then have the wizard Craft an Ogrehook with the same enchantments, how different is that?
The wizard had to take a feat, maybe we have to pay full price for the mundane weapon (unless a character can Craft those) and it takes some time. The result is the same.
Add a restriction that the spell fails on artifacts, and set the king's sword as an artifact, based on its legendary history, and move on. Isn't the King's Sword, with that legend attached, worth way more gold to the right buyer than a "no history" weapon with the same enchantments anyway? So, wouldn't a valid purchaser be very willing to swap for a freshly enchanted Ogrehook with the same enchantments?