If you are married, the [[Old Mariner]] will appear, but will not sell you a Mermaid's Pendant. If you get divorced, you can purchase another one, which can be given to any romanceable [[Villagers|villager]].
The Mermaid's Pendant cannot be sold. It may or may not be connected to the mermaids at the night market or the sea monster.
Note: The [[Old Mariner]] who sells it calls it an "old amulet".