Ok, here's my situation:

At some point in the past, my character looted Kagoruhn before getting the quest from Sul-Matuul and then sold the Shadow Shield to Wayn in the Balmora Fighters Guild. Wayn, of course, equipped said shield making it unavailable for me to purchase back. I can't even, as far as I can tell, pick-pocket it off him if he's holding it.

Any suggestions for getting it off him w/o killing him?

Sell him a better shield and the Shadow shield will go into his "sell" inventory.

Yeah...thought of that.... What's better? Tried selling him a Daedric Shield. Didn't work... Not sure exactly how the game is determining what's better. Is it giving weight to unique items?

Might work. If you say that Wayn equipped it, you must be playing on a box?

Playing on PC.

Ah, let's see what we can do. Open the console and type

player->additem "shadow_shield"

I hope it'll work.

I thought of that too... It should work, but seems like "cheating."

Sure, you could also reload an old save...

Ouch....pretty sure I did this a LONG time ago... So, I'd rather cheat than that.

I really like the idea of selling him something better, but I'm just not sure what would do the trick...

Yes, me too. I now remember why my characters used to store any item that would look precious in Caius' home. There were tons of them everywhere! Something you couldn't do in Skyrim without crashing the game... lol