The option to turn in items is really just to let you trade an item you dont need and get something for it. Its not really intended to be for farming seals, and it never was. This was the same in 1.0 also, and thats when GC gear was very high end gear. Now it's way easier to farm darklight (and AF2), so GC gear isnt as desirable (although its good to have when farming darklight).
The only time it was ever easy to farm GC seals in 1.0 was when they released Atomos (near the end of 1.0). And was just as mindless as FATE's.... Basically just show up at a specific time of day and a bunch of ppl would kill him, loot his corpse, trade the crystals he dropped for seals, rinse and repeat.
That being said, dungeons arent meant to be farmed for seals. They are meant to be farmed for items which are better than regular/gc gear.