The correct form is: τον ψαρά, τον πωλητή.
We definitely add “ν” in the male and female article, when the following word starts with:
1) a vowel (α, ε, ο etc)
2) the consonants κ, π, τ
3) the consonant combinations that include κ, π, τ (γκ, μπ, τσ, ξ, ψ). Please note that ψ and ξ come from the consonant combinations πσ and κσ respectively .
In the rest of the cases you may omit “ν”:
τη μαμά, το φίλο κτλ.
As a hard and fast rule you may always add “ν”. It is not a mistake to write:
την μαμά, τον φίλο κτλ.
However it is a big mistake to omit “ν” when it is required. We would never write this:
το αέρα, το ήλιο