Now that you're in the Guild, you're pretty much on your own. Your quest is called "Independent Thievery" and its whole objective is to sell stuff to your fence. But not just any stuff. You have to sell stolen goods. After joining the Guild, your fence is Ongar the World-Weary in Bruma - he lives in the little shack behind the chapel, but spends most of his day drinking with his buddies at Olav's or wandering the streets. As you complete other Guild quests, other fences will become available to you.
These fences have progressively more money, but they also have higher Mercantile skills. This means that you'll be able to sell more expensive items to your fence, but you're going to get a smaller percentage of the item's value unless you make sure that your Mercantile skill equals or exceeds theirs. Training, skill books, hard haggling with other merchants, spells, and enchanted items will all help.
I found a quick way to deal with all of the fencing in just about one fell swoop. As part of the Main Quest, after you break the Siege at Kvatch and take Martin to Jauffre, you go to Cloud Ruler Temple near Bruma (what a coincidence). If you have no bounty at the time, Jauffre will ask you to join the Blades. If you have a bounty, no matter how small, he will not make the offer. This is crucial because if you are a member of the Blades, you can't steal the stuff from Cloud Ruler (as a member of the faction, you can take what you want with no penalty). You can always decline the offer for now and join later, though. Anyway, there is a ton of stuff in the west wing of the Temple. During the day, there is only one guard in that part of the temple and he doesn't care if you close doors so that he can't see what you're doing. So just close the door, drop into Stealth mode, take everything that has a value (and there's a lot of it, including an unlocked jewelry box) and then saunter right down to Bruma and sell the stuff to Ongar. And even if you steal all of the Akaviri Katanas in the Temple, they'll give you another one when you join the Blades. All told, you should be able to fence close to a thousand septims' worth off of that one haul.
Some users have reported a problem with the final quest for the Thieves Guild that may trace its way back to this Independent Thievery mission. It appears that fencing significantly more than the required amount at your current rank can cause some script triggers to not set correctly, which breaks the last mission. I have not run into this problem, so I can neither confirm nor deny that this is the case, but be aware that others have reported it. Best advice: fence what you need to fence, but cut it off at about 1000 septims. Anything more than that might cause a problem. I'd hate for you to have to load an early save and start all over again.
An alternative cause of the problem may simply be that these players are not waiting for their journal to update before asking about the next mission. I haven't done a great deal of exploring in the Construction Set, especially with Quests and Journal Entries, but it's possible that the game is checking for how much stuff you have fenced before making most of the missions available, but is looking for particular Journal entries or Quest Stages in others. If you don't allow a few seconds for the game to advance the Quest Stage on this mission, it may be breaking the later missions. So play it safe -- when you complete a quest, get out of the conversation and wait until your Journal updates. It should be at least once, but possibly two times - once to tell you how much you need to fence for the next mission and once to tell you that you have fenced that much. THEN go ask for the next mission.
There are a couple of other options, too:
Once you've fenced an appropriate amount, you'll get a journal entry letting you know about special quests from the Guild and that you should go seek the appropriate doyen to get it. Here's a list of what becomes available and when. Unlike earlier Elder Scrolls games, your rank is more or less awarded as an afterthought for completing certain quests rather than as a mark of achieving any particular skill level(s). As long as you stay within quest parameters, it really doesn't matter how you accomplish the objective.
|Rank||Fenced Item Value||Doyen/
|Available Quests||Available Fence|
|- - -||
|Armand Christophe||May the Best Thief Win||- -|
|Untaxing the Poor||Ongar the World-Weary (Bruma)
600 gold - 60 Mercantile
|The Elven Maiden|
|S'krivva||Ahdarji's Heirloom||Dar Jee (Leyawiin)
800 gold - 70 Mercantile
|Lost Histories||Luciana Galina (Bravil)
1000 gold - 80 Mercantile
|Taking Care of Lex|
|The Gray Fox||Turn a Blind Eye||Orrin (Castle Anvil)
1200 gold - 90 Mercantile
|Arrow of Extrication|
|Boots of Springheel Jak||Fathis Ules (Elven Gardens District)
1500 gold - 100 Mercantile
|The Ultimate Heist|