they're sold all of the time, you just have to make sure it's done properly and documented, the concern is newer stuff and illegal trade of modern endanger species parts, most of these teeth were harvested during the height of the whaling trade in the mid-late 19th century..., most of the "new' teeth and scrimshaw that you see in shops is resin, the guy on Nantucket was selling stuff brought in illegally by a guy from the Ukraine, he'd(Nantucket guy)been in business for a long time which makes it hard to say he didn't know, not too many years ago you'd see these everywhere at antique shows/shops along with an occasional Narwhal tusk which is pretty cool, seems like the last 10 or so years they've really stepped up enforcement though the ban and most of the rules are from'73...many auction houses won't accept bids from certain states now, particularly California, Eldred's on the Cape will only sell them to Mass residents....Goosefish, if you have concerns and depending on the quality of the tooth which if it's large(bigger than 6 or 7 inches) you could be talking about significantly more money than the 1/300 I mentioned, just contact or take it to Eldred's or Skinner's
https://secure.skinnerinc.com/appraisals/form.asp, they deal with this stuff all of the time and have experts on staff, they'll tell you what it's worth and you can decide if it's worth selling considering the commission scale..the antique group shop that Nightfighter was referring to is Acushnet antiques