grayedout, I have one of the highseats shown in your ebay link, didn't buy it from the same seller but it looks exactly the same. They are fine but in all honesty, the panther looks better. They look to fold up more compactly for transporting and the ability to shoot sideways will pay premiums. First time I used my highseat I shot a red from a sideways shot and it involved coming out of the seat, crouching on the small foot platform and using the sloping diaganol of the shooting rail as a rest - far from perfect and I've now fettled it to have side rails too. I bought mine because I wanted one quickly and could pick it up locally, with hinsight I would have been far better off waiting and getting the panther delivered.
The seeland one looks good but the Panther has a few advantages over the other two. It is the lightest at 12.5kg, spares are available if you need them and lastly you can fit the extension which makes it a proper 'high seat'.