^^ It is probably illegal to scalp, but I always see lots of them at the Yoyogi venue, so maybe noone really cares much? But it'll be best if you get a Japanese friend to buy the ticket from the scalper instead of doing it yourself, since they may jack up the price because you're a foreigner and they know you need a ticket cause that's what you came to Japan for (to see ayu) lol.
So yeah, it's possible to get really good deals from scalpers, but just be careful!