The higher your speechcraft skill, the easier it is to convince people to do what you want through threats or coercion, and the better prices you get whether buying or selling.  All characters benefit, but thankfully the skill doesn’t require any special activities.

Level up your Speechcraft by buying and selling items, or by convincing or threatening NPCs.  As with any other skill, you can speak to trainers to pay for up to five additional ranks of the skill each level.

TRAINERS
Journeyman: Dro’marash (Khajit Caravans), Revyn Sadri (Windhelm)
Expert: Ogmund the Skald (Markarth)
Master: Geraud Gemaine (Bards’ College, Solitude)

PERKS

Haggling (1/2/3/4/5)
Buying and selling prices are 10%/15%/20%/25%/30% better.
Requires Speechcraft 0/20/40/60/80

Allure
10% better prices (both buying and selling) when dealing with a member of the opposite sex.
Requires Speechcraft 30, Haggling 1

Bribery
Can bribe guards to ignore crimes.
Requires Speechcraft 30, Haggling 1

Merchant
Every merchant will now buy every kind of item.
Requires Speechcraft 50, Allure

Persuasion
Persuasion attempts are 30% easier.
Requires Speechcraft 50, Bribery

Intimidation
Intimidation is twice as likely to succeed.
Requires Speechcraft 70, Persuasion

Investor
Can invest 500 gold with a shopkeeper to permanently increase their available gold.
Requires Speechcraft 70, Merchant

Fence
Can sell stolen goods to shopkeepers you have used Investor with
Requires Speechcraft 90, Investor

Master Trader
Every merchant in the world gains 1000 more gold to barter with.
Requires Speechcraft 100, Fence