We don't use "vendem um peixe", because they sell fish and not only ONE fish. If you use this you're implying that he sells just one fish.
Also, if you use "vendem peixes" (and you can use it!), you're implying that they have a couple of fish to sell.
Using the singular form "peixe" shows that fish is their field of business.
Hope you understand! Have a beautiful day!