This post is informative to me because my character just joined the Thieves Guild for the first time. Guess when I'm thieves guild master I'll dump the old uniform into a chest in my house or sth.

But well, I guess telling us players that certain items couldn't be sold because it's a quest item is kinda hand-holding to an extent. But if no such notification exists, at the least, I guess if we accidentally sold that said quest item, there should be SOME way to get it back (either go loot it back from where it's from, or in this case the armor, we could craft it since we're part of the guild anyways, etc.).
Well, I'm glad you won't fall into the same trap I did. I also assumed I would be able to at least kill off SOMEBODY to gain a piece of armor -- no cigar. You guys can sit here and talk as much [censored] about my choice, but I generally sell equipment I never plan on using. Especially since I knew I would be obtaining Nightingale Armor. And for all those people saying they wish there was no quest items period, thats great too, but at least they should be consistent. Clearly the Guild Armor was essential to completing the quest, and I also sold it to Tonila, so every conversation I have with her only leads to "Come back when you have a piece."
And in Oblivion, even if you tried, you couldn't get screwed like this. None of the major guild / quest lines were fully halted by losing something (at least in my 300+ hours on that game I never ran into this problem). Yeah, when I posted I was furious. How could I not be? I just spent over 100 hours on what I wanted to finally be my first perfect Elder Scrolls playthrough, and its all for nothing. If I had known, I would have just prepared for 2 playthroughs (like I did with Oblivion - did 2 guilds each time).
This forum is hilarious. Half the responses are just [censored] kids talking crap because I made a mistake that frankly, if the game were consistent, wouldn't have even been possible.
Thanks to benn1680 for the message.