If every item could be purchased with a single kind of currency, yes - it would be a Coinmaster, as Kolmafia defines it. But a single item - the tattoo - takes 5 different currencies, and therefore, we implement the shop as a creation method.
Grandma’s shop is also implemented as a creation method.
Both are implemented in KoL as shop.php and it might be nice to somehow mirror that model in KoLmafia, but that would be a lot of work for who knows what value.