There are a lot of things allowed on ebay and at live auctions only where they are over a certain age to be classified as genuine "vintage/antique"
I trawled back through 30 days of ebay things (all categories) which contained ivory. It seems that every one that was allowed to run to its conclusion specified a date of 1900 or before.

Certainly, in the UK, trade of ivory artefacts is legal if it can be proved they date before 1900 so I reckon the same rules apply.