Location: Nanohana
Petitioner: Mediator Marzin
Reward: 6,000 Berries, Mediator's Invitation
Objective Reward: Sandora Dragon Steak Tips
To trigger this Side Story you first need to gain access to Spice Bean Plaza, located in the north of Nanohana. This requires doing some back and forth running in the south of Nanohana until you have the Gate Key needed to open up the northbound street to the plaza. The petitioner for Mediator Marzin's Quest, Marzin himself, will be standing off to the right as you enter the plaza.
As a mediator Marzin's job is to accept Bounty Hunting targets captured by their fellow targets, such as the Straw Hats. He takes a cut and the bounty hunters get the rest. Accept Marzin's proposal and he'll tell you to check out the Bounty Board inside the Spice Bean, the bar in the north of the plaza. Do so and you'll find a contract for a guy named Evil Bulbro, who has been terrorizing the eastern road out of Nanohana.
Head east once you've accepted the Bounty Hunt and you'll find Evil Bulbro, along with his brother, in a short alleyway halfway along the road. They're eager to scrap, and clearly in over their heads.
Evil Bulbros
Weakness: Speed
These guys aren't total pushovers, but they are pretty easy to defeat. Both Bulbros use either normal physical attacks or a Skill that does a bit more damage. Their attacks can inflict Bleed status on your characters, and if you're really worried about that Chopper can step up and Heal anyone who might be in trouble. Otherwise, pull Nami and Usopp up front to deal with the Bulbros.
You'll earn an Evil Bulbro's Record Cube for defeating the Evil Bulbros. Marzin will appear shortly thereafter to give you your cut of their bounty - a measly 30,000 Berries - and speaking to him will unlock four more Bounties on the Bounty Board, back in the Spice Bean.