There are many different factions in Sarnaut that will keep track of your reputation with them. As you complete tasks for a faction your reputation with them will increase. The different levels of reputation are Hostile, Unfriendly, Neutral, Friendly, Trusted, and Respected.
You can view your reputation levels by checking the Reputation tab of your Character Info window. When you mouse over the bar for a certain reputation, it will show how much reputation you need to earn to reach the next reputation level.
Each faction has a quartermaster that sells high-quality armor and weapons. However, you will need to have a certain reputation level with the faction to buy the items. After you raise your reputation to the required level you will be able to go to a quartermaster and buy the reward you've earned.