Probably Pop Mart. A three-story shop filled with nothing but these odd little collectible toys of cartoon characters. It was absolutely mobbed.

Batteries + Bulbs.

They sold nothing but batteries and lightbulbs.

In Turkey we have shops that only sell pickles!

I have been in a shop only for set equipment for cooking book pictures

It’s not super specific, and makes sense for where I saw it, but I’ll never forget the Wisconsin tourist trap I drove past as a kid that has a big sign out front advertising “Cameras, Cheese and Guns”.

Cycling clothing. No bikes, no helmets, just more lycra than a gimp convention.

There’s a “craft”-art shop in the same cave I work in that exclusively sells racism.

I have not, myself, been there, but San Francisco has the Pirate Supply Store.

https://en.wikipedia.org/wiki/826_Valencia

826 Valencia is a non-profit organization in the Mission District of San Francisco, California, United States, dedicated to helping children and young adults develop writing skills and to helping teachers inspire their students to write.

826 Valencia runs San Francisco’s “only independent pirate supply store”. The store is the front entrance of the tutoring center and has the look and feel of an authentic pirate shop.[5] It sells pirate clothing, eyepatches, compasses, spyglasses, pirate dice, skull flags, and secret treasures. It features handmade signs, scattered around the store, offering tongue-in-cheek wisdom, such as “Uses for Lard” (#5: “Lard Fights”) and “Guidelines for New Shipmates” (#4: “No forgetting to swab”). Unsuspecting visitors are sometimes treated to surprise “moppings.”

Shoppers can also find back issues of McSweeney’s Quarterly Concern, books published by McSweeney’s, and literary compilations written by 826 Valencia students. The books of student writing published by 826 Valencia include forewords, illustrations and interviews from public figures such as Former San Francisco mayor Gavin Newsom and actor Robin Williams.

The Pirate Supply Store was originally created through necessity. After being turned down for regular use of church basements and school facilities, the founders discovered an empty store on Valencia Street. City ordinances required businesses in that area of the city to be either retail or catering, so the Pirate Supply Store was opened in 2002 as the “legitimate” business front for the writing center. It also helps fund the tutoring program.[6]

There was a store in Vegas that was magnets. Fridge, city, anything magnets.
And sardine stores in Portugal, airports and elsewhere.

I went into a store that specialized in selling canes when I visited London.

Kite store in Chicago (it’s really not that windy there, really)

Synthesizer store. RIP to Seattle’s Patchwerks. Such a damn shame they went out of business.

I came across a very narrow, 5 story store in Japan, a bit off the beaten path in Akihabara, selling exclusively N scale model trains. They had a floor or two dedicated just to the models, another one for scenery, and at least one just for repair parts for your trains. It was really cool to get to see a store that was pretty clearly a passion project for the owner(s) apparently doing well, and filling quite the niche in the hobby (I imagine)

Rubber stamp store. Just shelf after shelf of rubber stamps and rubber stamp accessories.

I used to live across the street from a ballbearing shop.