Just checked out all the stuff on Ongar the World-Weary (the lowest level fence that lives in Bruma). He's in the faction called "Fences", which may have something to do with it. It doesn't look like anything special in the Factions window, though. Doesn't actually seem to do anything at all!
Try adding him to the Thieves Guild, too. Since only Thieves Guild members are fences, there may be some line of script or something somewhere that makes it so that only they are.
If you look into the 'MerchFence' class, nothing there says they can or cannot buy stolen goods - therefore it must be something else. Proves that you've done nothing wrong so far, but doesn't really help to determine what's missing.
It might just be worth duplicating Ongar, then editing his face, AI, inventory... etc. until you've got what you wanted. That's just a last ditch attempt though!
I'll carry on looking into it, and tell you if I find anything conclusive. He seems pretty ordinary to me so far! Good luck, though.