Roughly speaking - if you mark an asset lower than its purchase price you have an unrealized loss. You still hold the asset and - if you're taking the super long-term investment horizon view as Buffet does - eventually your asset will go up. In addition, Buffett gets into investments that probably wont have their business models disrupted anytime soon - and are tied to the success of the USA as a country: railroads, insurance, housing.If he does his stock picking right, he doesn't need to sell...