First, on the legality of sealskin sporrans:
Yes, Native Americans and Aboriginals can hunt fur seals and use their skins to manufacture things (the seals' skins, not the NAs'). They can also hunt whales (within limits), possess bald eagle feathers and talons, and use peyote in religious ceremonies, all of which are highly illegal for us latecomers. Not being critical or racist; just stating some facts.

In the United States, North American fur seals are protected by the Marine Mammal Protection Act, which is enforced by the US Fish and Wildlife Service and US Customs (the Men In Blue). There are also South African fur seals, which are protected under the International Convention for Trade of Endangered Species (CITES). There are also populations of seals and/or sea lions in Scotland; I don't know if these are protected or not.

Knowing this, here's roughly what will happen if you try to bring sealskin into the US (I can't speak for any other country). If you try to bring it in personally and declare it, it will be confiscated by Customs to turn over to Fish & Wildlife. If you don't declare it, and it's found, it will be confiscated and you will be spanked. If you try to mail it, it will be caught at a Customs' interntional mail division, where it will be confiscated (and you may still be charged duty). Thinking UPS, FedEX, or DHL - they still have to declare all international shipments to Customs, and if they get caught mismanifesting something (like forgetting the "sealskin" part of "sealskin sporran"), then they can get some serious fines levied against them - and Fish & Wildlife will take your sporran.

There are alternatives - I know McRok ships bovine hair sporrans to the US that look like sealskin.

Second, on the sporran itself, and I apologize for not posting this before Robin bought his. I have handled one of the ones confiscated by US FWS (the FWS inspector didn't know what it was ). The opening under the cantle was barely over three inches across, and the whole sporran (including cantle) was about nine inches in length. There was no gusset between front and back, so there was no room for expansion. I know it's supposed to be a dress sporran, but IMHO those dimensions are too small for it to be functional.