The Pawn Shop Planet is a very tiny planet, most likely a dwarf, that appeared in the episode Raising Gazorpazorp. It is a planet with nothing but a Pawn Shop on it. The ground of the entire planet is a parking lot, made out of pavement with nothing but a Pawn Shop and two lamp posts. It is the smallest planet seen in Rick and Morty, even smaller than Dwarf Terrace-9.
It's only known inhabitant is an alien who most likely doesn't live there but inky goes there to work who is known to eat sulfur. Given the fact that no vehicle other than Rick's space cruiser was seen on the planet, it may be possible that he lives there, or it's possible that his car was just on the other side of the planet, invisible to us, at the angle it was shot at.
This could be a reference to one of Roiland's old shorts(Fantastimart), that can be seen here: http://roilandtv.com/fantastimart-01-and-02/