It's pretty much your fault if you sold your armor. Why anyone would sell such a great armor (especially for thief characters early in the game) is beyond me.
I guess you just don't deserve to be the Guild Master then.
This, I would never sell any unique item, that is items who I suspect either looked unique or had unique bonuses,
Why sell it in the beginning it would give you huge bonuses for thieving actions, later you don't need the money.
Now I can understand it happens with an accident, myself has sold crafting gear, like smithing bracers,
Now I find all the un-droppable quest items a pain, yes because it is seldom stated in the quest log that is quest items it's hard to avoid it, one idea might be an mark like star in front or special color not for sale and a prompt if you remove it would do nice.
In wow you can not sell quest items and you are have to type delete to drop them but you can transfer them to the bank.