In betrayal in Antara you can select skill just like spell domain. At first i was thinking it was for passive skill up like spell domain but even if i rest a week, there is no skill up. So i was wondering what was the point of selecting a skill domain like defense or haggle?

IIRC, you only improve when you actually use those skills. E.G. the only skill you are focusing on is defense, you get into a fight, and artificially extend the fight & let your opponents repeatedly attack you then defense should increase more than if you were not focusing on it at all.
I'm not sure, but haggling may on work only the first attempt after entering a shop. (At least in BaK.) Also, focusing only on a single skill gives significant diminishing returns according to the manual.

I can't comment on BIA OR RTK, but I can offer advice on raising various skills in Betrayal At Krondor.

Haggling: Go to the Yabon shop. Mark only the Haggling Skill for aOwyn, Locklear, and Gorath.
Note: A successful haggle will get you a faster increase than an unsuccesful haggle, but both will increase your skill. Also, the Haggle affects all members of the team, even if only Owyn haggles, so Locklear and Gorath will benefit from Owyn's work. Yabon's haggling number is 50, so you won't get many successful haggles until your Owyn's skill reaches 51.

Haggle all the items in the shop. If the shop owner agrees to a reduction, decline it, and move to a new item.
If the owner doesn't agree, try again.
If the owner doesn't agee and makes the item unavailable, move to a new item.
When all the items have been haggled, leave the bargaining room, come back in, and begin again.
I usually take Owyn's and Locklear's haggling skill to 88 (Gorath is always with Owyn so his haggling skill isn't important.

Lockpick: Go to the locked chest near the Die chest in LaMut. The lock's skill number is 16, do a lockpick skill of 17 is required to open the chest. Locklear's skill is 25, but he'll always be paired with James, who is better skilled for locks.
Gorath's lockpick skill is only 15, and you only have 2 lockpicks, so don't break any.
Note: whoever has the highest lockpick skill is always pick automatically (Locklear), you have to manually select Gorath until his skill gets to 26. The higher your skill gets, the more sucessfull picks have to be made.
Get Gorath's lockpick skill to 88

Weaponcraft: Go to the Dimwood South and grab the 8% Goblin Sticker out of stump. Go to the Yabon Shop. Sell the Goblin Sticker, and buy it back for $23.40. Whetstones cost $2.00. Have Gorath and Locklear buy, repair, and sell Goblin Stickers until both of their Weaponcraft skill reaches 80. By this time, you'll have made $10,000.

Armorcraft: Don't repair the 18% Standard Armor you started the game with. Use it in combat until it drops to 10%. Get hammers from the Yabon shop and go to the LaMut shop and sell the 10% armor. Buy it back. Have Gorath and Locklear repair and sell armor till their Armorcraft Skill reaches 95. You won't make much money, but you no longer need it either.

Crossbow Skill: Go to LaMut and buy 2 Elf Crossbows for $638 each, 9 Lt. Bowstrings for $17.60 each, 600 quarrels for $10.60 for every 25 quarrels. Head north to the Trap with the Sorcerer in Tyr-Sog. Both Locklear and Gorath will fire quarrels at the Sorcerer, but red and green poles block them. Their crossbow skill will still increase, so get both of their crossbow skills to 88. When a crossbow breaks, repair it with a bowstring and keep shooting quarrels. Once their skills are at 88, keep shooting until 1 of the bows break. Then kill the Sorcerer and escape.

Casting: Use the trap in the Dimwood, or the one south of Zun to increase Owyn's casting skill to 80, by casting 3 point Gift of Sung spells at Gorath or Locklear.

Barding: Only Owyn needs a decent barding skill so buy Lutes in theTanneur shop and increase as desired.

Melee: Use the 2 Scorpian endless combat (Ockie's house) in the Dimwood NW. Get Locklear's melee skill to 95.
Note: Locklear is the one to focus on for Melee and Defense, as he leaves after Chapter 1, and he'll need to have 100% skills in Melee, Defense, and Crossbow to deal with the enemies he faces when he next appears.
Use starving to bring Locklear's health down to 21, give him Dragon Plate Armor with a #3 Blessing. Give him the weakest sword possible. This will ensure his weapon strikes have 11 or less points of damage per strike. The less damage, the more hits can be made, and the more Locklear's Melee skill goes up. One Scorpian will attack Locklear, and the other will attack Gorath. Use the Grief spell to freeze Locklear's enemy, as he is vulnerable. Let Gorarh defend only, and let Locklear kill both Scorpians. Note: During your first combats in the game, always focus on using only Locklear (if possible) for all your melee attacks.

Defense: Bring Locklear's Defense to 74 using the Brak Nurr. Then get training from Devon in Eggley for 15 points, and Psalms of Dala for 6 points. Naturally, Ockies endless scorpian combat could be used, but it probably won't be necessary.

Finally, use Thiful's Book on Bird Migration to finish taking all your important skills to 100%.

In Chapter 2, you can use Sumani, Tad in Questor's View, Ockies endless scorpian fight, Lord Lyton's Tax Collectors, Abdul, etc to bring James up to speed.

I can't think of anything else, but hopefully this will help a couple of people out...

Previous posters are correct about skills advancing by using them, but, IIRC bia is a little weird when improving magic. The best way to improve magic skill is by abusing the free room and board privileges at the first inn (family owned one if not first). Anytime someone is seriously injured or even poisoned, don't use medicine and antidotes to revive them. Haul them back to the inn and let them near-death from poison and then revive super slowly by massive resting.

As for why you would focus on haggling or lockpicking for example, it is useful to reduce the repetition required to level it up. I'm more familiar with BAK, which I would generally focus defense/scouting/stealth while walking around. The simple reason being scouting occurs with no notice and stealth checks can too. After defense is 100, just focus scout/stealth for general travel.

In combat, looking at your inventory wastes a turn, but adjusting skill does not. So in combat, you can wear inadequate armor (or remove it entirely eventually) and focus only on defense. Eventually opponents will be near dead simply by -1 hp per 'swing' instead of thrusts. When they start trying to flee they will never attack again. So swap to focus on whatever method of attack you want to focus on and then kill them. As a minor side note, if you use the left wall and have your team N, NE, and E of an opponent they will never move. You can rest your entire team to full health and then kill the enemy however you like. I don't think there's an "invitation" equivalent spell in BIA, which makes that trick harder to setup.

When you aren't in combat you should focus on the skill you are about to use next, be it haggling, smithing, or whatever. Particularly, do this when you are using a skill trainer or a skill book. Trainer improvements are heavily influenced by skill focus.