Charisma

Charisma improves the buy/sell prices at Space Stations for both items and trade goods.

In addition, for every 33.33% of Charisma, you can have +1 more crew member to follow you on planets.

Charisma has no effect on any other kind of shop (such as Terramart) or prices of Trader NPCs.

Sources of Charisma bonuses and penalties

Armor
Bloodhound Armor (5% per piece, with a bonus 2.5% per level when upgraded)
Gold Armor (15%, set bonus)
Diplomat Armor (25%, set bonus)
Troll King Armor (-100%, set bonus)
Clothing
Nonpareil Shirt, Nonpareil Pants, Hawker's Top, Hawker's Trousers, Toga Top, Toga Bottom (5% each, with a bonus 2.5% per level when upgraded)
EPP Augments
Charm Necklace (20%)
Broken Master Manipulator (20%)
Racial
Kirhos have +25% to charisma ("Great Traders" bonus), while Pharitu have -10%, and Mantizi and Thelusians have -25% ("Poor Traders" penalty).
Other
Madness can have an unpredictable effect on how Charismatic you appear to others.
Successfully maximizing your investment in a Trade Station will also provide a trade effectiveness boost to all stations you interact with, up to a max of 10 invested stations.
The Douche Hat penalizes with -20% charisma. Anyone willing to benefit from charisma should avoid this item like the plague.