Fathis Ules is the highest ranking fence for the Thieves Guild. You can sell items to him for up to 1500. He also spends much of his time in Dul gro-Shug's house, but you can not sell stolen goods to him while he is there unfortunately.
You can sometimes find Fathis in or around his home, opposite the King and Queen Tavern, in the Imperial City Elven Gardens District. He's also been seen walking around in the Imperial City Market. Sometimes if you wait in or at any of these locations, you'll be able to catch him before he leaves again.
Sins of the Father requires you to recover a sword stolen from Castle Chorrol. After retrieving the sword you have the option of giving the sword to Fathis for a substantial amount of gold or returning the blade to Castle Chorrol. Should you decide to return the blade to the castle, Fathis will remain in Chorrol and forever shun you, and you will be unable to use him as a fence. However, If you finish the quest by selling him back the sword for 1000, you can then legally buy it back off of him for 1600. It still weighs nothing! This saves weight and as it is ebony, it is a good weapon so you will not need another sword for a long time. Note: If you buy back the sword, you cannot give it back to the castle for the shield.
Note: Fathis' travelling and fence behaviour can be unusual and unpredicatble. He has been known in both the X360 and PC versions to stop offering fence services for no reason, and has also been known to never be in either Chorrol or the Imperial City. The most common, and worst, glitch involves him never leaving Chorrol during the course of Sins of the Father. There are unofficial modifications (mods) available that fix his behaviour If you're using the PC version. Unfortunately as of right now, though, the Xbox360 version of the game does not have an update for this problem.
- I have found several times that if you follow or find him sitting on a bench in the Elven District, the Fence option comes up. For some reason you cannot access the Fence option if he's walking around in or outside his house.***
If in the Sins of a Father quest you gave the sword back to Fathis then from here on he will tell you that "Our business is done" and leaves the converstaion making it impossible to buy or sell anything from him. this is especially irksome if you are following the thieves guild quests and have him as a fence.
You may also try waiting a few days after going through Redguard Valley Cave, and then asking him about the quest.