Over the years I have bought and sold single and small groups of picture post cards from WWI and WWII. How much they went for depended mostly on the picture, more than what was written on them. The ones with military related pics and/or war scenes always brought the most. The ones with men in uniform with weapons and pics of equipment and tanks/artillery pieces always brought the highest price. High price being $5 to $10 . I have also had the travel type fold-ups and booklet postcards of foreign publication that showed war damage or battlefields with destruction, that when annotated by the sender, brought interest. I currently have a group of travel booklets and photos, some with comments, that were bought and saved by a vet that was part of the occupying army in 1945-46 in Europe. I recently showed then to a dealer at a show, but he wasn't to interested in them. I think this area of collecting has a small following that may be best found on ebay.
Good Luck, knifefan0