(Lazy to get a screenshot because I can't get the Treasure Map to show up in the Black Market when I wanted it x_x)
When you buy a treasure map from the black market merchant, it always show the quantity of maps that you have as 0. After buying the item, it would later show to be 1. However, this is not accurate.
Despite how many treasure maps you have that remains unclaimed, the black market merchant do not count their quantity. This might be because of how treasure maps are not an actual item in the inventory that has a fixed value (summoning fragments, for example).
Not anything game breaking, but worth noting it's existence nontheless. I'll get a screenshot whenever I can, but it's pretty easy for anyone to see that this exists. (i.e. the forum lurkers ;P)