Hi All and thanks for your kind answers ! @Violadamore thanks for pointing me towards the good rules for posting on this forum. I indeed found rather strange that a violin maker would run the back of a violin this way; I thought it might be to mark the violin for a school or something like that. Here is the link to the auction, which seems to surprisingly have quite some people wanting it. I am not sure it's such an easy re-sell, so some may be attracted by the black cross, weird.
By the way, is there any reason why one can post only twice a day ?