Cantom - glad you posted this, it's something that's good to be discussed, and something I've addressed at an academic level on several occasions, so feel that I'm at least able to contribute a little to the discussion.
History is something to be understood. Items with historical lineage are more interesting because you can place that item at a location or point in time. Having items from the past connects you to the event. I don't approve of anything the Nazi's stood for or did, but having items produced and used in the Second World War, same as the Great War is important to me, regardless of which side they belong to, because they are a direct connection to the past.
It seems a lot of people have a hard time separating recognition of the past with admiration of it. In many ways something physically from a period/place in time is a lot more telling than any book can be, and often makes one's knowledge of the time a lot more real, or factual, than if one is trying to imagine it.
If the seller is using Auschwitz provenance as a selling point but has no way to back it up, then it's a bit bad taste. If he wants to use that as a selling point he should provide evidence, at which point it becomes a fact. Whether it is ok to profit from something like this is open to debate. On the one hand practically everything in our society has a value, and rarer or more emotionally loaded items command a higher price. On the other hand there is a moral point at which profiting becomes reprehensible. Like with most things, there is no defined line, just a gradient, and where that gradient lays is different for different people.
So, in conclusion I understand that people have strong reactions to things like symbols or terrible events. However, it is my feeling that letting those feelings force you to completely ignore or suppress all knowledge or connection of those events and utterly cut any association with them does the people involved more disservice than remembering and acknowledging them in a respectful manner. I feel that without providing proof of provenance, this ad fails to do so, but is most likely not done with any intent to harm.