The I.R.S. has long viewed the valuations given for artworks in income, estate and gift tax returns as a “potentially high abuse area,” as described in several recent reports. It sometimes uses a group of dealers, museum curators and scholars, known as the Art Advisory Panel of the Commissioner of Internal Revenue, to advise the agency on works worth more than $50,000. According to annual reports by the panel, about 58 percent of the 1,840 works it reviewed from 2011 to 2015 were valued improperly.