The name of the sellers or the buyers of the masks remain a secret (where are they all coming from?). If America was to implement the 1970 UNESCO Convention properly (with an export licensing procedure for such artefacts) then there would be no problem with stopping the sale of those that left the US after the date it was instituted. The problem is that the US does not want to protect this cultural property in such a way.
An auction on Monday of sacred masks and objects in France has stirred fresh anger among Native Americans, with representatives of the Navajo people travelling to Paris to try and halt the latest sale. The Eve auction house has 270 Native American, Eskimo and pre-Colombian artefacts going under the hammer and the United States embassy has stepped in, urging a stop to the sale of items cherished by the Navajo and Hopi people. The sale is the fourth since 2013 that the southwestern Hopi people have tried to block of ceremonial masks and headdresses they consider to embody living spirits. All previous legal efforts to halt such auctions have failed, although a US foundation last year bought 21 of the masks at a Paris auction to return them to the Hopi people.
What's left of Hopi and Navajo
Indeed it is rather ironic that when a group of people from precisely the area occupied by these tribes were caught stealing artefacts, including sacred ones and robbing graves to obtain collectables, they got sentences no greater than a few months probation and the authorities that investigated, arrested and charged them received a lot of criticism for the way they did so. It seems that illegal activities within the US concerning artefacts of precisely this region is not treated with any great seriousness.
Afp, 'Native Americans try to block French auction of sacred artefacts', 14 December 2014.
UPDATE 15th December 2014
Navajo officials won their bid to buy back seven tribal masks at a contested auction of native American artifacts in Paris that netted over a million dollars. Monday’s sale went ahead despite the best efforts by the U.S. government and Senator John McCain to halt it. The objects for sale at the Drouot auction house included sacred masks, colored in pigment, believed to have been used in Navajo wintertime healing ceremonies. The sale — which totaled 929,000 euros ($1.12 million) — also included dozens of Hopi Kachina dolls and several striking Pueblo masks embellished with horse hair, bone and feathers, the origins of which are unclear.[...] The lawyer representing the absent Hopi tribe, Pierre Servan-Schreiber, said the approaches of the Hopis and the Navajos were different — and said the Hopis see the sale as sacrilege. "Hopis were opposed to buying back their artifacts as they did not want to engage in the auction," said Servan-Schreiber.Thomas Adamson, 'Navajos buy back artifacts at Paris auction', Salt Lake Tribune 15th December 2014.