I think your dealer is right since "marca da bollo" means "revenue stamp" in italian Stamps like yours were used in all former Italian Colonies and many of them went overprinted during British Occupation. https://www.revrevd.com/libya-briti...upation.html
I've contacted the owner of the site you shared, Andrew, and he suggested that "This is a revenue stamp issued for the Italian Colonies in Africa (ie Eritrea, Somalia and Libya) around 1930."
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