Kourindou is an antique store seated between human civilization and the Forest of Magic, run by Rinnosuke Morichika.
The store itself is used as anything but. As a somewhat well-known establishment for folks in the world of Touhou, Rinnosuke rarely (if ever) actually sells anything, despite keeping an exhaustive inventory - he mostly trades and/or puts up with "creative borrowing."
On occasion, the store is used for altogether non-business ends, such as cooking and combat. The store also takes on a bit of damage over time, thanks to conflicts that involve the store's occupants at some particular time.
The inventory of Kourindou is (presumably) its main draw, as it mostly includes items from the outside world. Teacups, feather pillows, computing magazines, kerosene heaters, and even an iPod make its way there over time.
As Kourindou is a side story (東方香霖堂 actually follows a somewhat parallel timeline to the games), no character in a game has actually visited or mentioned the existence of Kourindou yet (as of 東方花映塚 Phantasmagoria of Flower View).