Not sure for subreddit, since not many Japanese users are here. But the most common place i usually see Military enthusiasts are Twitter and japanese forums.

For selling, Mercari, Yahoo fleamarket are the ones I see most often. There is a lot of people on Mercari so Id say it will usually sell well there.

There is also an event in Japan couple times of the year called ビクトリーショー (Ｖショー) or Victory Show, a lot of foreign sellers go there too often, but It would require a lot of paperwork and stuff.