To begin the 31. Item Swap side quest in Trinity Trigger, speak with the Junkmonger in Grimgreed. For this quest, you’ll need to deliver his wares to a merchant on Stahl’s Main Street. This merchant can be found just southwest of the shrine in Stahl Kingdom: Main Street.
Once you deliver the items, return to the Junkmonger in Grimgreed to complete the quest. As a reward, you’ll receive 6000 Liba and a Cracked Doom Mask.