Hello from Japan!
 

Hi guys, i'm a 24 year old ex marine living in japan for the ski season, The season comes to an end in April and i still have a year left on my Visa and i don't really want to leave the country so early, i'm huge into my bikes and have a BMW G650GS Sertao back in England waiting for a RTW trip when i return. I was just posting to find out if anyone knows of the legalities of buying a bike in japan to travel the country before selling it when i leave in October time. I have previously bought bikes in Laos and Vietnam with ease, but i am well aware the laws over in SE Asia are very relaxed and even problems can't be wavered with $10 note. I'm certain its not the case over here. Any information would be much appreciated, Many thanks. Sam.

As long as you have legal residence, you can buy a bike.
You will need your Certificate of Residence (Juminhyo) in order to register the bike in your name.
You can probably find more information on Gaijinriders