Is permissible to buy/sell with non-Muslim?
Yes buying/selling for a Muslim with non-Muslim is permissible as regards what is lawful.
What is the ruling on looking at pictures of naked women via the internet?
All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
It is impermissible to look at pictures of naked women via the internet, and this act is forbidden according to Sharia. The proof of this is that Almighty Allah Says (What means): "Say to the believing men that they should lower their gaze and guard their modesty: that will make for greater purity for them: And God is well acquainted with all that they do." [An-Nur/30]. Therefore, we advise you to busy yourself with offering acts of obedience that draw you closer to Allah and prepare yourself for meeting Him on The Day of Judgment. And Allah The Almighty Knows Best.
Is it permissible for someone to make a supplication to Allah to marry an already married, or an engaged woman?
No, it isn`t, and the person who does so is sinful before Allah, The Almighty.
Are dream interpretation books accurate?
All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon Prophet Mohammad and upon all his family and companions.
Dream interpretation books are unreliable because they are merely expectations, not rulings of Sharia. However, if one sees a dream which one does not like, one should spit on one's left side and seek the refuge of Allah from the satan; it will not do one any harm, and one should not disclose it to anyone. If one sees a good vision, one should feel pleased but should not disclose it to anyone but whom one loves. And Allah The Almighty Knows Best.