I have a few different items in my inventory where the description reads something along the lines "sells for a modest amount".
Items examples are mysterious ore and fools gold.
Are these items safe to just sell or do they have any other kind of purpose?
No, there are no other uses for these items. Without writing up an exhaustive list for each item, there is no way to be absolutely sure - but it is safe to say that each of these items are in the game for the sole purpose of selling for currency.
This is a common concept in many other RPGs, as well. For example, the Gold Nugget in Pokémon games.
