@Chanak: Unfortunately Duriel has a Holy Freeze aura at Nightmare and Hell, same as the Pally skill, and that is magic cold, why Cannot be frozen does not help. Nothing helps against Holy Freeze. Cannot be frozen is however good against many other monsters, especially in Act VI.
Item drops are random, although if you wear a set item you have a higher chance of getting another item from the same set. At least in theory. I always end up wearing 1736 not so good set items (you have to have them equiped) because I want another item from those sets, but I don't think it has ever worked
The higher level the monster has, the more likely it is that it drops a better item, and you can only wear items with Magic Find to increase your chances of getting Magic, Rare, Set and Unique items.
@Silver Knight: I gamble as soon as I have plenty of money. The odds aren't very good, but as you character increase in level odds to get good items improve. The best stuff to gamble are IMO boots, gloves and lower level weapons. Armour, rings and amulets are seldom worth it since they are so terribly expensive. Remember though that Magic find does not affect gambling. In each act there is a fix level of how much money you get by selling items to the shops, and the limit is 10k in act II normal, just as you have noticed.
Heh, Ancient Kaa the Soulless
He is my favorite monster in the entire game, he can be tremendously difficult
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