I actually like this because it gives me an excuse to donate something, but I agree that containers should be sellable. (Not even bags are sellable right now.)
Maybe a vendor will offer the price of the bag and everything inside it when you drag a bag into the sell window?
You can auction or trade empty bags, but right now we're prohibiting the selling of bags because there are a number of technical challenges involved in enabling it. We may revisit this policy down the line, but right now there are plenty of more important things we would like to focus on. Basically, it's not that we don't want to allow it from a gameplay perspective, it's due to technical limitations.
Is it because the bag is an item made up of a list of other items, and the system isn't set up to deal with selling multiple items at once right now? ^^