I've sold a couple of park give-aways that were printed on postcard stock, postcard size, clearly intended to advertise/promote a place. I listed them in the appropriate Post Card category and made it quite clear they were not postcards (mine had info printed on the back and could not have been used for postcards). Nobody questioned or complained, and they sold for what I expected.
I also have a couple of RPPCs of women holding their baby, clearly non-commercial cards, one has the usual Post Card back, one has just a dividing line. It seems to me if your cards were meant to be postcards it would have been so easy to mark them, the omission indicates they weren't meant to be postcards.
The postcard stock and machines to print/handle them sure make it attractive to use them for non-postcard items!
Jim in KC
Jim in High Ridge