The Marine Mammal Protection Act of 1972 imposed a moratorium on the taking and importation of marine mammals in the United States. Section 102 of the Act makes it illegal "for any person, with respect to any marine mammal taken in violation of this title, to possess that mammal or any product from that mammal."
The exclusion that supposedly allowed the sale of seal skin rests in interstate commerce was this one:
"Except as provided in section 1379 of this title, the provisions of this chapter shall not apply with respect to the taking of any marine mammal by any Indian, Aleut, or Eskimo who resides in Alaska and who dwells on the coast of the North Pacific Ocean or the Arctic Ocean if such taking—
(1) is for subsistence purposes; or
(2) is done for purposes of creating and selling authentic native articles of handicrafts and clothing: Provided, That only authentic native articles of handicrafts and clothing may be sold in interstate commerce: And provided further, That any edible portion of marine mammals may be sold in native villages and towns in Alaska or for native consumption. For the purposes of this subsection, the term “authentic native articles of handicrafts and clothing” means items composed wholly or in some significant respect of natural materials, and which are produced, decorated, or fashioned in the exercise of traditional native handicrafts without the use of pantographs, multiple carvers, or other mass copying devices. Traditional native handicrafts include, but are not limited to weaving, carving, stitching, sewing, lacing, beading, drawing and painting; and
(3) in each case, is not accomplished in a wasteful manner."
I use "supposedly" because I'm skeptical that the sale of seal skin rests ever satisfied this provision. That skepticism is apparently shared by all the national retailers that I know of who formerly sold the rests but have ceased doing so.