「MONEY, MONEY, MONEY」
In my JoJo universe, gods exist to some extent, and can claim mortals they deem worthy as their champion. This champion will be granted a stand that is somehow related to the domain of the god that sponsored them (though in JoJo fashion, it is not usually as direct as something like a storm god simply granting control over weather for example).
3M is a stand belonging to the champion of Cissonius, the ancient Celtic god of commerce and travel.
Ability: 3M allows the user to summon a shop containing 3 random items, arranged from cheapest to most expensive. The shop can be summoned as often as he wants, but every day it resets, containing different items. The stand allows the user to buy these items, so long as he can offer the appropriate value; meaning, not limited to just currency. At the same time the shop resets, any items he purchased that day will be lost. The catch is that credit is not allowed. He can only pay with valuables on hand, that he can reasonably claim belongs to him. Keep in mind the user of this stand lives in medieval times, when paper currency was not widespread.
As said previously, the items are arranged cheapest to priciest. The cheapest item is quite affordable, and usually the user has enough value on hand to purchase it. Because of this, it's also the least helpful of the 3 items. It might be something like a spool of rope, a bucket of apples, maybe a knife if you're lucky. It can definitely be useful, but you'll have to be creative.
The moderately priced item is a step up. It could be a nicely forged sword, a perfectly tuned guitar, or something like that. Also, this is when the shop stops being limited to physical items. You may be offered something like a contract for a single guard, which upon use summons someone to protect you for a time; stuff like that.
The expensive items are where the stand gets interesting. The amount that these items cost would likely be inconvenient to constantly carry around, but they are worth it. The expensive items are often enchanted, maybe sometimes replicas of famous mythological items. But they can also be more utility-based. Maybe you can purchase a contract for a large house that begins building itself magically upon use, or you can buy a safe and fast travel across the sea. Perhaps you could buy a magic fishing rod, that upon being used in water summons a massive sea monster.
Of course, the most expensive item is typically the one you want the most, but gathering the goods for it will not always be easy. This can force the user to either work with the less expensive items, or think of creative ways to gather the resources necessary for purchase (internal organs are valuable, for example).
There's one more layer to the stand: if the user hasn't opened the shop yet that day, he can request a Special Shop. These Special Shops affect the price and quality of the items. For example, he could request a premium shop, where every item is powerful, but very expensive. Or he could do the opposite and request a bargain shop. A fun one is the mystery shop, where each item is priced the same, but their contents are hidden until bought, and he can only make one purchase.
Appearance: upon summoning the shop, a skeletal figure in black robes stands silently, the 3 items displayed in front of him. If you've ever played Hades, it's a lot like Charon's shops. The keeper is intangible, and so are the items until purchased.
If I win, I'd like the next theme to be stands that permanently affect the user's body.