Welcome to Lineage II :D
1. The PvP/PK system is very complex in Lineage II. You might want to take a look at the basic guides (
http://www.lineage2.com/guide/game_guides.html) as well, although the official guides are more theatrical than helpful (the player written are much more helpful).
Basic PK/PvP-- you can attack other players in non-peace zones by holding the ctrl button and then the corresponding F1-F12 skill slots. Not all spells will work-- some spells, such as Root and Sleep cannot be used as an initial attack. After you have attacked someone, your name will turn purple and will remain purple until you stop attacking and wait 20 or so seconds in which your name turns white again. As long as your name is purple, you have "flagged" and are eligible to be killed without your opponent becoming chaotic/red. If whoever you are attacking does not attack back and turn purple themselves, then if you kill them, your name will turn Red and you gain Karma. As long as you have under 5 PKs, if someone kills you, you have a low chance of dropping items. If you have above 5 PKs, then you have a very high chance of dropping items.
However, if both you and your opponent attack each other, you will both turn purple. When someone is killed when they are purple, they have a very low chance of droppping items but still lose exp. The person who killed them does not turn red.
You can attack anyone anywhere except for peace zones and some other areas. The arena is a good place to practice-- in an arena, players do not flag (unless you run out of the arena as someone is attacking you), drop items, or lose exp in the arena for fighting.
2. You can only buy up to mid C grade in stores. All items under that type are purchaseable from store NPCs. Beyond that, everything else is crafted by players.
Non-grade: 1-19
D grade: 20-39
C grade: 40-51
B grade: 52-60
A grade: 61+
S grade: 75+
Items can be enchanted in Lineage II, although a risk is involved. When you use enchant scrolls on an item, its stats get boosted slightly. When you plus something beyond +3, then there is a 1/3rd or so chance that your weapon will simply explode into crystals. After +3 weapons begin to glow as well, and their stats are improved at a higher rate. The more glowy the weapon, the higher it is plussed. Weapons begin to grow red at +16, although there are very, very few weapons beyond C grade that are +16-- there is such a high chance that a weapon will explode somewhere in between that people won't take the risk of losing all that money from buying the enchants.
Weapon and armor value are pretty straightforward, and aren't set by the game. Rare things aren't always good, and good things are always rare. If something is harder to craft or acquire, then players will usually sell it for more.
Box of Fox
P.S. We can answer many many more of your questions, and based on how I answered them I'm sure you'll have more. Ask away :D Oh and from now on you might want to use the Player Questions forum instead of the Guides one. (edited: nevermind, your post got moved anyway)