Tonilia

Tonilia is a Redguard thief and a member of the Thieves Guild in Riften. She acts as an early fence for stolen items and occasionally sells lockpicks. Without perks, she has a 1000 spending limit, which resets every 48 hours. That amount increases to 4000 after becoming Guild Master.

Tonilia's side quest
You unlock her side mission once you are done with 3 of the 4 special jobs for the Thieves guild. Her side mission is quite simple, although somewhat necessary because if incompleted, it will be difficult to acquire small jobs for the last city you need. Once you are done with your 3rd Special job, speak with Tonilia, she will say that she needs you to do something for her. She says she needs to expand the importing business in Skyrim, through the Khajiit. She will ask you to take Moon Sugar to their leader, Ri'saad. He is directly outside of Whiterun, near the Whiterun Stables. You lead a one way conversation, and he agrees to help the Thieves Guild. After which he becomes available as a merchant, and his caravams become additional fences. Return to Tonilia, and she gives you 800 gold. Easy, simple mission.

Bugs
If you sell or lose your starting thieves guild gear, then upon receiving the "free upgrade to a piece of your choice" option, all future interactions with her will result in a dead end discussion of upgrading the gear and you will be unable to fence anything with her anymore.

As of Patch 1.4, Theives Guild armour of all kinds is found around the Rataway - Cistern which Tonilia will accept if you loose your first set, meaning she will upgrade it, get new dialogue, start fencing again and you can actually complete the Thieves Guild questline. It is unknown whether THESE sets respawn or not, so don't lose THOSE either! Tested on PS3 and XBox.

This could potentially be worked around by using the console to spawn replacement gear.

There are three sets of Thieves Guild Armor you can spawn through the console, the ones you want are as follow:

player.additem 000d3ac2 1 player.additem 000d3ac3 1 player.additem 000d3ac4 1 player.additem 000d3ac5 1

There are NPC versions used by NPC thieves guild members who are not lootable and will not work for this sake. The full list of items are

00036584 - Armor, NPC version

000d3ac3 - Armor, Basic version

000d3acc - Armor, Upgraded version

00036583 - Boots, NPC

000d3ac2 - Boots, Basic

000d3acb - Boots, Upgraded

00042be9 - Glove, NPC

000d3ac4 - Glove, Basic

000d3acd - Glove, Upgraded

00036585 - Hood, NPC

000d3ac5 - Hood, Basic

000d3ace - Hood, Upgraded

Ran into a bug where after completing Taking Care of Business, Tonilia won't act as a fence and will only buy non-stolen goods - Reloading an earlier save fixed the problem.

Glitch: Tonilia will not upgrade your Thieves Guild Armor after the "Loud and Clear" quest.

Trivia

 * Several conversations that take place in The Ragged Flagon state that she is in a relationship with the owner of The Ragged Flagon, Vekel the Man, though both seem to have different views as to where they want the relationship to go, in a conversation between the two, Vekel states that he wishes she would slow down and live with him, while Tonilia states that she is just fine with the way things are. In another conversation between Tonilia and Dirge, it is stated that he believes Tonilia is cheating on Vekel with Brynjolf, though she quickly denies this, telling Dirge to mind his own business.