Well states have various laws in regards to this, in many of the midwestern states it is not against the law to sell or possess purchased wildlife parts. In NC, there really is a grey area, it is legal to sell items that were created from wildlife parts but the individual part cannot be identified as such and cannot appear to look like 'said' part. It is illegal altogether to sell meat except that from raccoons and opposums if i recall correctly.
Supposedly in NC, one cannot buy or possess bought wildlife remnants and cannot purchase same over and across our NC state line. However the laws are grey in color and one cannot readily find such written General Statutes readily available for our knowledge.
That is how i understand it to be at present.
Now, there are game farms out in the midwest and even north of us in the states that allow it like Texas, Michagan and others that raise and sell dropped sheds off of these game farms. It is perfectly legal to buy these type of antlers. They are classified as 'preserve animals' i suppose by the fish and wildlife departments and open up this area for the sell of the antlers from these registered game farms.
More than likely the individuals that are selling in the 'spotlight' on ebay are owners of and managers of these wildlife ranches that sell dropped antlers.
Chris