I looked at YahooAuc yesterday. I think almost all the tickets I saw are for classical concerts, and priced below the retail price.

I also stumbled across this resale site. Prices are actually a bit below retail, but since fees are hidden I suspect they will be pretty close to actual price once everything is factored in. No seat numbers.

https://www.viagogo.com/Concert-Tick...810?quantity=2

Good tips here:
https://www.japanconcerttickets.com/resale-tickets/