What does Selling Fish reward?

Selling Fish gives Murloc Fin Soup, Recipe: Murloc Fin Soup, Fishliver Oil outright, 10s, 1,250 experience.

Where does Selling Fish start?

Dockmaster Baren in Redridge Mountains offers it.

Its level has not been recorded yet; it shows once a player accepts it with the addon running.

No. The quest exists in Classic Era as well.

World of Warcraft: Forever · quest

A quest in World of Warcraft: Forever, offered by Dockmaster Baren in Redridge Mountains. Recorded by 6 players running the addon.

Level and objectives show once a player with the addon accepts the quest.

Item	Given	Item level	Vendor sell
Murloc Fin Soup	outright	10	1s 25c
Recipe: Murloc Fin Soup	outright	25	4s
Fishliver Oil	outright	21	68c

What does Selling Fish reward?: Selling Fish gives Murloc Fin Soup, Recipe: Murloc Fin Soup, Fishliver Oil outright, 10s, 1,250 experience.
Where does Selling Fish start?: Dockmaster Baren in Redridge Mountains offers it.
What level is Selling Fish?: Its level has not been recorded yet; it shows once a player accepts it with the addon running.
Is Selling Fish new in WoW Forever?: No. The quest exists in Classic Era as well.

Source: what players running the addon saw in the quest windows; the quest text itself stays in the game. Every quest seen · Redridge Mountains