[MMM]

I can't say whether or not there would be a demand, but I think your second question is the important one. Even if you had 1000 masks to sell, where are you going to sell them and who are you going to sell them to? You are not a licensed distributor so stores won't buy from you. You could set up a table on the street somewhere, but I don't know if people want to buy sterile medical equipment from a 15-year-old foreigner when the disease itself came a foreign country.

Buy a handful and bring them as omiyage and enjoy your stay in Japan.