Saeviomagy said:
Believe me, they'll do it. And stopping them from doing it will require a lot of the DM saying "no, that just won't work" without much in the way of justification. Much better just to, say, make the doors highly decorated, and assign a value to the decorations? Unless you really want the focus of the campaign to shift to "what do we do with these big valuable doors we have".
Actually, I think players trying to get away with this certainly is the fuel for a number of adventures.
First, they have to figure out a way to break down the doors into a form they can carry through the narrow passages that lead to the area. Additionally, they have to decide how they are going to carry all that metal (in whatever form they end up with). They also are going to risk a lot of encounters in the area (you don't think other rats, swarms and the like won't wander into the area) during their "salvage" operation.
Second, they have to figure out how they are going to get it out of sewer level of Dorasharn tower. The inhabitants are very poor and anything that seems valuable enough for the attention the PCs pay to it is very likely to get their attention. It's not like they are going to be able to hide carry that amount of metal through the area. They are going to draw
a lot of attention carrying that much through the area. Especially if they have to make several trips through the area.
Remember these are first level characters. The commoners in the area could give them a hard time, not to mention the rogues and warriors floating in the immediate area. Plus, the areas surrounding it aren't the highest class areas of Sharn, either.
Finally, they have to "fence" it. Almost anyone who would be interested in it will have ties to House Cannith. They are either going to be dealing with very, very questionable people, or else they are going to risk House Cannith finding out what they did. Making an enemy of House Cannith (even just the local branch) is really not something the PCs should be interested in doing. Doing so will make Sharn a very hot place for the PCs, and outside of Sharn isn't going to be cake, either.