You can do a couple of things with your assets when you close a business. The items could be: sold, retired, stolen, destroyed, disposed of, converted to personal use, traded in, given away or it's no longer being used in the bsuiness for some reason. These are options you have when go to your business assets and you see the page "Tell us more about this asset/large purchase" (see screenshot).
Remember to go through the Business Profile to indicate the business is no longer. (see screenshot).
If you have other questions about this, ask in the comment section below.