But you cannot buy motorbike on tourist visa, you need at least edu visa or now some trustworthy Thai

I brought are car last week. Had the Blue book changed to my name, no problems. I am on a Tourist Visa.

I intend to buy a second hand scooter. I've a triple entry tourist visa.

What's the green book I keep seeing?

When I buy a scooter - what do I have to do in order in terms of registration/tax?

I've my own insurance already.

The Green Book is the registration document for a motorbike/scooter (Blue book is for a car).

You will need to get a Certificate of Residence from Phuket Immigration. Phuket Immigration will require some sort of proof of your address in Phuket, usually a rental contract supported by the owners ID and house registration. Plus copies of your passport.

You will need the transfer form signed by the owner, your address in Phuket, signed by you. Plus more copies of your passort. Best the seller or owner accompanies you to the Vehicle Registration Office. The bike is subjected to a test/check at the vehicle registration office, frame number checked against documented number in book, the vehicle's value is assessed, and a small transfer tax has to be paid, plus the fee for registering your name in the book.

You will still need to buy the compulsory governemnt insurance to be legal in Thailand. Small cost, something like 350 baht/year.