Oblivion:Mercantile

Mercantile skill governs haggling, which allows you to sell goods to merchants for more while buying for less.  It ties in with Speechcraft.

In-game Description: Get better deals when buying or selling goods.

Mastery
Once your Mercantile skill reaches 100 (without any fortify mercantile enhancements) it is possible to buy or sell items at 100% of their base price. Otherwise, you will always sell items at less than 100% of the base value of the item, and buy at more than 100%.

Mastery Perks
The gold received by a Novice for goods is reduced because of wear.
If you have an Expert Armorer skill, repairing armor or weapons to 125% will actually increase the barter value of the item, up to the base value of the item, but not over.
An Apprentice no longer has prices reduced due to wear.
If you have an Expert Armorer skill, this can actually cause items repaired to 125% to decrease in value compared to the same transaction conducted at novice level.
A Journeyman can sell any goods to any vendor.
Vendors can also sell any goods to a Journeyman of Mercantile.  This unlocks a few unique items carried by merchants who don't otherwise sell items of their type.
This does not allow you to sell stolen items to a non-fence.
This does not allow you to sell quest items.
An Expert can invest money in a shop to permanently increase the shop's available gold by 500.*
For a Master:
All shops in the world have 500 more gold for barter.  This is in addition to the 500 gold you may have invested in a shop.
All transactions with all shopkeepers can be done at a glorious 100/100 ratio (buy and sell items at base cost).  This has been confirmed in the patched version of the game.
This bonus is not obvious. The barter slider continues to show buy/sell ratios that are worse than 100/100, and if you push the slider too far to the right, a merchant may still refuse the transaction.
  
Skill Increases
You gain 0.4 experience points per sales transaction, regardless of the number of items being sold, the value of the items, or the setting of the barter slider.  You do not gain any experience for buying items, except for spells (for which you gain 0.4 experience points per spell bought).  You also do not gain any experience for giving away zero-value items.

To increase mercantile skill, you will want to sell items one by one, as this gives you experience for each item, rather than 0.4 experience for the batch.  Arrow stacks are a very good example on something to sell in this manner.    The Select One control shortcut (LB on Xbox 360; L2 on PS3) is useful for quickly moving the "How Many?" slider when selling items individually. Even so, increasing mercantile skill can be very tedious.  At higher levels it is well worth spending money on Trainers rather than spending hours selling items one at a time.

If you have not installed any patches, the Kvatch infinite money glitch also provides mercantile experience every time you sell your weapon.

See also:
Increasing Skills: General tips on how to improve your skills semi-naturally.
Trainers: Pay for mercantile lessons.
 
Skill Books: Books that provide lessons in mercantile.
Increasing Attributes: Tips on training skills to improve the Personality attribute.
Free Skill Boosts: One free mercantile skill boost (for 5 points) is available by completing the quest A Shadow Over Hackdirt.
Magic Items: There are multiple generic items with the fortify mercantile enchantment.
Unique Items: A list of unique items, some of which fortify the mercantile skill. These do not include Leveled Items such as those received from quests, or Artifacts.
:Unique items include:
::*Boots of the Swift Merchant are a unique pair of boots that can be obtained from Seed-Neeus at the Northern Goods and Trade when your mercantile skill is over 50 (otherwise she does not sell armor). Among other enchantments the boots fortify your mercantile skill by 10 points.
::*Imperial Breeches are a pair of unique pants that can be obtained from Thoronir at the Copious Coinpurse. Among other things the pants fortify your mercantile skill by 5 points.

Bugs
 

Dialogue
When your skill reaches 70 or higher, NPCs will start saying: "You look like a shrewd businessman."

Character Creation
The following races provide initial skill bonuses in mercantile:
+5 bonus: Redguard
+10 bonus: Imperial

The following standard classes include mercantile as a major skill:
Agent, Bard, Healer, Pilgrim, Rogue, Thief

Merchant Disposition
If a merchant rejects your offer, it will lower their disposition. Since this affects your ability to get a good deal, it's best to start low and gradually bump up by a few notches than start at the top and work your way down.  An exception is when buying spells. A rejected offer will not affect their disposition.  For merchants who sell both spells and items, this can be exploited: determine the best possible barter rate from the spell menu (where there is no disposition penalty), then switch to the item menu.

A merchant's disposition will rise by 1 for every 100 gold either lost or gained by the merchant in a single transaction. Selling an item worth more than the merchant's available gold will result in a disposition bonus determined by the amount of gold you got for the item, not the amount the item is worth.

Investing in a merchant to increase their available gold also increases their disposition by 10 points, but does not otherwise affect the barter slider.

Tips
Once your combined Fame and Infamy reaches 40,  you can visit the Sithian heaven doomstone to receive the greater power Sithian Web.  One of several benefits of this power is that it fortifies your mercantile skill 20 points once a day for 120 seconds.
You only need to raise your Mercantile skill and disposition long enough to activate the merchant, so a spell with 1-second duration Charm on Touch and 1-second duration Fortify Mercantile on self is all you need.
You may still invest in a shop after reaching Master mercantile, allowing for a total increase of 1000 gold in any shop or fence you choose. These two perks (investing to get 500 more at Expert skill, and automatically getting 500 more at Master) are independent of each other.
 
 
Since the looting and selling of items for profit is an essential gameplay element for many players (regardless of character class), you may find it difficult to control the leveling of the Mercantile skill, especially if Stealth is your specialization and Mercantile is one of your major skills.
If money isn't a problem for you at the moment and you wish to control the leveling of this skill, one solution is to store all of your collective loot in your house or a safe container (such as the inn room drawers in Aleswell Inn after completing its related quest) and then sell the items whenever you wish to level the skill.

Messages
The following table provides the messages that are displayed when your Mercantile skill levels up.



Notes
 

For more details, see the commerce page, and this spreadsheet showing merchant comparison curves and break-even points and other details.