It would depend on the intent.
A museum owning an SS emblem, obviously for educational purposes, is fine. A creepy fascist who owns an attic filled with Nazi-glorifying material is breaking the law.
An online auction of Nazi material would be breaking the same laws, I suppose, as long as it took place on Dutch territory. I know, hard to determine, in this case.
To put things into perspective, it might be useful to know that in The Netherlands (which I generally regard as one of the most liberal and free-thinking countries in the world), Mein Kampf is, to this day, and illegal book. You may not own, nor sell a copy of it.
Also, bringing the Hitler salute in public can result in a prison sentence.
Yes, we take it that serious. I completely understand that the French verdict may look somewhat outlandish from an American perspective. But just try to imagine the local factors. There are still lots of WWII victims alive. And even if there weren’t, it would still be considered important to send out the message that recent history has been very grim.