Can I react to an opponent moving Traveling Merchant to an open battlefield and using their effect with ACTION spells, or does it need to be a REACTION?

Ruling: You cannot play ACTION spells in response to Traveling Merchant being moved and using its effect. You can only play REACTION spells during those chains. You can play an ACTION spell only after you get focus.

Sequence:

• Opponent moves Traveling Merchant (move trigger occurs BEFORE showdown)
• During the move trigger chain, only reactions can be played
• Showdown begins
• If there is an initial chain, only reactions can be played
• If opponent plays an action, only reactions can be played during that chain
• THEN you get focus and can play an action

Nuances:

• Actions can only be played when you have focus, not during triggered chains or in response to opponent actions