So, is it possible for a non-Thieves Guilder to sell stolen stuff to anyone? I've yet to find a merchant who would buy the items I stole. Not even Enthir at College buys those even though I'm the Arch-Mage! He even seems to try to hide his Guild contacts from me (and he's doing it poorly: "Certainly nothing elusive going on here, Arch-Mage!" ). So, who'd buy the poor stealing Arch-Mage's stolen goods? Or should I just join the Guild just because of this?

I made a topic of this before but never got real help: what's with Onmund? I've done his quest and unlocked him as a follower. Yet the "Follow me" dialogue option has vanished. His greeting line to me has also changed into "You're nothing but trouble." Has his disposition towards me lowered for some reason? I haven't stolen anything from him (that he knows of, at least) or done him harm in any way. Brelyna still follows me normally at least. J'zargo did, too... until he disappeared. I might've accidentally killed him during our travels, but I don't know where it might've happened and I never saw the body. I wasn't kicked out of the College, either. Could it be that Onmund is angry at me because he knows I killed J'zargo (even if I don't myself)? But Brelyna isn't? How to fix Onmund to be a follower again? I'm on PC so console is my friend.

There's a Speech perk called Fence which will allow you to sell stolen goods to merchants you've invested in.

So it seems - thanks! Although I don't want to invest any perks to Speechcraft, at least with this character. And it's not like Fence was the first perk or anything like that at all. I find the whole perk tree for Speechcraft pretty dumb actually, mostly because getting too much money is easy without it anyway. I'd rather join the Guild and roleplay myself more as a part-time associate / College Arch-Mage with some extra privileges than an actual member.

I could've sworn I read there's an all-access fence somewhere. Might've been a bug and fixed with one of the patches. Let me try to track it down though. I'll edit in a few.

Crap. Sorry. Couldn't find anything promising. Can a spouse buy stolen items?

I don't know... I haven't tried. I joined the Guild already, though. Still would appreciate help with Onmund.

As for Onmund, I'm just guessing here, but if this is a disposition problem, I'd try this:
setrelationshiprank 3
where is Onmund's ID (just click on him with the console window open). 3 stands for "ally", but lower numbers (0-2) should be enough to set his disposition as friendly, too.

If that doesn't work, this code adds him as a potential follower:
addfac 5C84D 0

If nothing works, try removing him from Follower faction with
addfac 5C84D -1
and then add him again.

If that fails, I'd probably go for disable -> enable. We'll see.

Nothing you suggested worked, but thanks anyway. D: Onmund stays without his third dialogue option.