UNPOPULAR OPINION: Wurt shouldn't be able to trade with pig king, but not after a tweak to the merm king
1: Have normal merm king trades have a chance to give gold (About 10-35%, idk)
2: Have trinkets give gold
Like pig king, trading trinkets will give you varying amounts of gold, depending on what trinket it is
However, the merm king will give 1.5x the gold the pig king does for that same trinket
3: If you try to trade meat, merm king will hit you with his trident, dealing 150 damage