Businesses can deduct the full cost of new and used qualifying assets with lives of 20 years or less that they buy and place in service during the year. A used asset can not have been depreciated by the previous owner.
So, for all practical purposes, the asset purchased cannot be real estate, or attached to real estate. No buildings or components of a building. A used asset purchased cannot be bought from a business unless the asset was in inventory for resale. So, used assets purchased from individuals or businesses that sell used goods qualify.