Well, three days with no replies. I doubt that these are considered collectible as such. I created some for use with the first day of issue of the 2015 Coast Guard stamp. Of no philatelic value by themselves, my rationale was that they might FDC collectors might view them as contributing to the "cachet" of the covers I affixed them to. I've not tried to sell any of them, so I have no idea whether it would add to the interest of these FDC's or not.