On Oblivion
Mercantile is the skill of buying goods for less and selling them for more. It's governing attribute is Personality.
 Skill Perks
Apprentice: No longer has the price of goods reduced because of wear.
Journeyman: Can sell goods to any type of vendor.
Expert: Allows you to invest money in to stores that will permanently increase the available gold by 500.
Master: All stores in Cyrodill have at least 500 gold to barter with.
You gain 0.4 exp for every item sold(this doesn't count for 0 value items). To train this skill you'll want to sell each item one by one. When selling items in bulk you can use a shortcut to quickly select just one item (L2 on the PS3, LB on Xbox 360). Tips for training are;
- Sell every item you collect one by one. If you're a member of the Fighters Guild and Mages Guild then you may want to take all items of value available just to sell them.
- Using Alchemy to train your mercantile can work very well when selling all the potions you make one by one, especially if you're master in the skill and can create potions from just one ingredient. You'll train you're alchemy in the process and will make good money.
- Buy a large amount of iron arrows and just sell them one by one. It's the quickest way to train it but it is the most tedious. You'll only lose 1 gold for each arrow you sell.