The Fight Merchant is the 5th chapter of the Rurouni Kenshin manga.
Somewhere in Tokyo, a man is fighting a group of thugs, knocking them to the ground easily. As one of the thugs admits his and his companions' defeat, the man calls the fight boring and asks if there are any "real fighters" in the vicinity.
Back at the Kamiya dōjō, Kaoru is happy that she found an ink painting from her late grandfather, which she can sell for a lot of money. She tells Kenshin and Yahiko that they ought to celebrate this and go have some sukiyaki.
The three of them then end up at the Akabeko restaurant. While they are waiting for their food and drinks, the restaurant's manager, Tae, gets punched by some thug, himself part of a group of rowdy individuals.
However, before Kenshin and Yahiko can intervene, the fighter that we saw earlier catches Tae, preventing her from falling, and then picks a fight with the thug who hit her.
And so, in the street in front of the Akabeko, the fighter defeats the thug in a mere flick of his finger, right after taking a suntetsu (a hidden metal spike about 6 inches in length) to the forehead without suffering any significant damage. One of the thug's friends is about to go help him, but gets subtly threatened by Kenshin, and so him and his companions end up leaving, carrying their defeated friend on their shoulders.
The victorious fighter and Kenshin then have a brief, friendly exchange. After Kenshin refuses to fight him, the man gives him his address and leaves without paying his tab at the Akabeko, the Kanji for evil clearly outlined on his back.
That night, the fighter is eating in some household, presumably at the address he gave Kenshin, and is talking with the Hiruma brothers. Addressing the fighter as Zanza, Hiruma Kihei reveals that the one called Kenshin is actually Hitokiri Battōsai. This piques Zanza's interest, and he thus resolves to fight the Battōsai with some weapon that he hasn't used in years.