The mark next to the stamps is from the American Philatelic Experting Service--who authenticated the stamps and the post card as authentic. The post card is designated as UX27; the card first appeared in 1914 and was available until 1951. The three stamps in that condition are today considered as common; 807 is the 3c; 815 is the 10c and 831 is the 50c. They all are from the Presidential Series of 1938. However, the combination of the stamps, the card, the authenticated impression, and the Certificate should warrant a few bucks; certainly worth a try if you sell.