Among the many mysterious items often littering a number of magic shops of places where these huge clamshells...
...clamshells of sizes able to hold a fully grown human of tall build coming in often odd colours (for a clamshell) that is said to be able to turn humans into merfolk so long as all the requirements or instructions were followed.
first is that water is required. see that in order to do a transformation the clamshell must be submerged in depths corresponding it's size to perform a transformation and as such those who gain possession of such a clamshell must own a pool or aquarium of considerable size to make proper use of the clamshell.
second is that the user's tail colour is based on the person and neither the clamshell or clothing worn during a transformation (WARNING: any clothing worn that contradicts to merfolk tails is destroyed during the process), as such one must be accepting of getting a tail colour contradicting to the favourites.
third is that one must dry the tail off outside the clamshell to revert to human form, the only way to remain a merperson forever to to never get dry.
fourth for transgender users (pre or post op) the gender you identify as is the merperson you become. example a pre-op boys to girls will become a mermaids from usage but revert to boys when dry. post ops need not worry about reverting to old gender when reverting.
fifth that pregnancy is possible for female users (unless transgender like above) and babies born in transformed state can become human like parents when dry. however if children born as one gender after getting legs starts identifying as the other later in life then when they use the clamshell it will reflect the change.
Now then who is buying or getting such a clamshell: