You can find all sorts of outdoor gear and clothing just be aware that the products for sale change with the season. They stock everything From hiking boots to surfboards and if you look carefully you might find a good deal. These days the prices in second hand stores in Japan has gone up significantly however, sometimes you get lucky. And if you just need something like an old backpack, these places usually have some good quality cheaper options.

Tip: there is a Xebio sports store next door where you can buy new stuff and compare prices.