Na it's a mistake by the developers....
The game teaches us that quest Items lock... we learn that quest essential Items cannot be sold by the game...
Although it might seem foolish to sell unique armor it's perfectly reasonable that a player might not want to keep it.
Bethesda dosent want anyone to be punished for making decisions in the game.... mabey you do... but Bethesda has demonstrated with skyrim that that they want the game to be an enjoyable experiance for everyone... not just the hard-core rpg'er.
Agreed. I'm quite surprised and saddened by how personally some players look down on other players who *gasp* sell useless items. This isn't real life...it's a video game. I have zero sentimental attachment to my thieves guild armor and they made zero indication that it would be any more important than any other piece of armor in the game. This is an obvious glitch/bug/oversight on the developers' part. In the highly unlikely scenario that the developers actually intended this, they would have told you so. Brynjolf would have explicitly said "You sold your thieves guild armor, and thus you obviously don't care enough about the guild to be master." If that were the case, many players would have been angry, but the developers' intent would have been clear. Instead, we have a broken dialogue tree that simply cannot complete
Quest A because
Insignificant Side Quest B is not complete. Some people obviously feel pretty proud of the fact that they hold onto every shiny object they find in video game, but that does not change the fact that his is an issue that should be resolved like any other. I really hope they include it in the next patch. It is literally the only major issue that I had in my 150 hours of gameplay (aside from the issues that were introduced by the 1.2 patch).