Some online stores offer a discount on a customer's first purchase — what is the ruling on creating a new account each time a customer makes a purchase in order to obtain that discount?
All praise is due to Allah, and peace and blessings be upon our master the Messenger of Allah ﷺ.
It is obligatory upon a Muslim to uphold honesty in both his words and his actions. Allah the Almighty says {what means}: "O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know." [Al-Anfāl/ 27] And He says {what means}: "O you who have believed, fear Allah and be with the truthful." [Al-Tawbah/ 119]. And the Messenger of Allah ﷺ said: "Whoever deceives us is not of us." (Reported by al-Bukhārī.)
As for creating a new account for the same buyer, the terms and conditions of the website must be observed and respected. If doing so violates those terms, it is not permissible. And Allah the Almighty knows best.
Is it permissible for a wife to leave her house without asking her husband?
It is impermissible for the wife to leave her house without asking her husband except for a sound reason.
What is the ruling on preparing food for someone who is not fasting in Ramadan?
It is forbidden to prepare food for someone who is breaking their fast without a valid excuse in Ramadan.
Doing so would be assisting in sin, and assisting in sin is itself a sin.
Allah says {what means}: "but help ye not one another in sin and rancour" [Al-Mai`dah/2]
Is it permissible for a wife to refuse to go to bed with her husband (for sexual intercourse)?
It isn`t permissible for her to do so unless for a sound reason.