A Moderator is not part of the Dev team. It is called "Thieve's Guild Armor" and you didn't think to hold onto it. Even when they imply that you need to hold onto it... I'm not rehashing this argument again, but it is entirely the players fault that you sold it.
It is most certainly not the player's "fault" for breaking a questline, and that's a key point that you appear to be casually disregarding here. Selling the armor results in a
broken questline. Not a failed questline. A broken one, and Bethesda wouldn't
intentionally set up a quest to just break. If you want to argue that they actually would (as at least one other person in this thread has), then you're certainly not doing them any favors. Nor are you doing players any favors when you argue it's their "fault" that a quest broke.
I managed to break this questline by doing something else entirely. I didn't sell the armor, but I failed to complete the guildhall rennovations before playing out the main quest. This resulted in "Under New Management" not starting, even after I'd finally completed the guildhall upgrades. I suppose that's my fault too, right? For not immediately realizing that guildhall interior decoration must take precedent above all things.
And apart from the above, there have been numerous bugs reported in regards the Thieve's Guild questline. Several of them quest-breakers. They'll fix them in due course, I'm confident, but in the meantime, making excuses for the devs and/or shifting blame to the players in regards to an either hastily-produced or poorly-tested quest (or both) is completely wrong-headed.