goldfish peddling; goldfish peddler
Refers to both the act of selling goldfish and the person who does it. The context usually makes the meaning clear.
Long ago, goldfish peddlers walked through towns selling goldfish.
I saw a goldfish-selling stall at the summer festival.
Compound of 金魚 (goldfish) and 売り (selling, seller), from the verb 売る (to sell).