If I see bad-looking girls in the market? What is my sin?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Whoever deliberately looks at a woman who is not Halal for him (who is not one of his Maharim) then he has indeed committed a sin and exposed himself to the punishment of Allah, The Most-High, as Allah Says (interpretation of meaning): {Tell the believing men to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts, etc.). That is purer for them. Verily, Allâh is All-Aware of what they do.}[24:30].

In addition to this, Ali (Radiya Allahu Anhu) said that the Prophet (Sallallahu Alaihi wa Sallam) told him, “Do not have a look (at a woman) and follow that look with another one, as you are not held accountable for the first look, but you are held accountable for the last one”. [Reported by Ahmad, and Al Arnaut said, 'that is a sound Hadith'  (with other supportive evidences).  Jarrir also narrated: “I asked the Prophet (Sallallahu Alaihi wa Sallam) about a sudden glance (at a woman), so the Prophet (Sallallahu Alaihi wa Sallam) ordered me to take my gaze away (immediately)”. [Reported by Muslim]

Allah knows best.