Lunar Plane, Homebrew

The Shop
A large bright-blue glowing portal appears, taking the players to an alternate dimension if they so choose to enter. It is breathable and possesses gravity the players are accustomed to.

Inside the portal is an extra-planar pocket dimension, containing a single natural stone path to a small cave large enough for medium or smaller creatures, while the rest is empty black space. Entering the cave, it contains natural elongated stalagmites chiseled at the top used as pedestals for unique items, specifically of lunar rarity. In the middle of the cave, a single black creature of indeterminate size and shape sits idly on top of a much larger stalagmite base, covered in a soft blue glow. The creature can speak to the players if asked a question, but is otherwise unnaturally silent.

The creature appears to only accepts lunar coins as currency for the provided items and is omniscient while in the pocket dimension; all sleigh of hand rolls in an attempt to steal merchandise from the pedestals or them will automatically fail. If an attempt is made, punishment for theft or harm against it results in confiscation of all stolen goods and a temporary banishment from the cave, as an ethereal non-magical barrier shuts the only entrance to the shop.

The same spot the players entered the extra-planar dimension also contains a returning portal that will transport them to the spot where they initially entered.

Lunar Coins
Lunar coins are small silver coins with a peculiar ethereal blue outline, a small emboss decorating the edges. Lunar coins are a unique resource typically obtained when a confrontation (combat or otherwise) with another creature who possesses lunar items concludes; however these coins can also be distributed in other ways through loot pools and as quest rewards if desired. Outside of the Lunar Planes, these coins hold no real monetary value within any economical system, though this would not discourage collectors and scholars from trying to obtain them for whatever purpose.

Given the scarcity, these coins tend to be given individually with the same scrutiny as precious gems, only obtaining one or two per encounter.