Developing websites for selling haraam things
Question

I am a web developer, and I recently developed a site for a client. This client sells CD's, he has a music store here in the US. I did give him tips on how to promote his site on the net .
Is it okay that build and help this person setup his web store? .
Praise be to Allah.The basic principle in sharee’ah is that when Allah forbids a thing, He forbids its price. Singing and music are haram and are not permissible, because of the hadith narrated by al-Bukhaari, according to which the Prophet (peace and blessings of Allah be upon him) said: “There will be people among my ummah who will permit adultery, silk, alcohol and musical instruments.” The evidence that singing is haram has been discussed elsewhere – see questions no. 5011 and 5000. 
If it is clear to you that these things are haram, then it is not permissible to help people with haram things, because Allah says (interpretation of the meaning): 
“…but do not help one another in sin and transgression” [Al-Maa’idah 5:2]
You have to seek that which is permissible and good, and avoid haram work or helping others to do that which Allah has forbidden, because the Prophet (peace and blessings of Allah be upon him) said: “Allah is Good and only accepts that which is good. Allah enjoins upon the believers that which He has enjoined upon the Messengers. Allah says (interpretation of the meaning):
“O (you) Messengers! Eat of the Tayyibaat [all kinds of Halaal (lawful) foods which Allah has made lawful (meat of slaughtered eatable animals, milk products, fats, vegetables, fruits)] and do righteous deeds. Verily, I am Well‑Acquainted with what you do” [Al-Mu’minoon 23:51]
 “O you who believe (in the Oneness of Allah — Islamic Monotheism)! Eat of the lawful things that We have provided you with, and be grateful to Allah, if it is indeed He Whom you worship” [Al-Baqarah 2:172]
 Shaykh Dr. Khaalid ibn ‘Ali al-Mushayqih
 We advise you not to work with the owners of this website, because the money that you get from developing haram websites is considered to be haram money and this is haram. The Prophet (peace and blessings of Allah be upon him) said: “There is no flesh that is nourished by haram things, but the Fire will be more entitled to it.” Narrated by al-Tirmidhi, 558; classed as saheeh by al-Albani in Saheeh al-Tirmidhi, 501.