The people who still own an NES Zapper that is in good condition can find willing buyers for the peripheral in Japan, as the international version of the gun is worth a lot to Japanese audiences, due to the popularity of the Splatoon series.
The Nintendo Entertainment System was usually bundled with an NES Zapper and a copy of Super Mario Bros./Duck Hunt, with the latter game being compatible with the gun. It's for this reason that both games are still on the list of the top twenty best-selling games of all time. The NES Zapper was compatible with a number of games, like Hogan's Alley and Wild Gunman, some of which were launch titles for the system. The version of the NES Zapper that was released in Japan looked totally different from its international counterpart, as it more closely resembled a real revolver, rather than a futuristic raygun.
The international version of the NES Zapper has shot up in price in Japan. According to Chris Kohler from Kotaku, the NES Zappers are selling for around six thousand yen (over fifty dollars) on second-hand sites.
The reason why the NES Zapper has become so popular in Japan is because of the Splatoon franchise. There is a weapon in the Splatoon games called the N-ZAP '85, which looks just like the international version of the NES Zapper. The '85 in the name is also a reference to the year in which the NES was released in North America. As such, the second-hand NES Zappers are referred to as N-ZAPs in their description.
The Splatoon franchise is huge in Japan, to the point where it sparked a shortage of Nintendo Switch units when Splatoon 2 was released in the country. It's crazy to think that a fairly common kid's toy from the '80s is increasing in price like this, which just makes us regret not taking better care of our old Nintendo and Sega stuff from when we were kids.
Source: Chris Kohler/Twitter