I'm purchasing a motorbike from an individual right before they leave Thailand. What do I need to get from the seller besides the bike and the green book? Do they need to sign the green book or any other document? I don't want to find out the I missed something after they have left the country.

Always best to go to the land transport department (LTD) with the seller. There can be problems with the bike that can complicate the transfer.

You need signed a completed transfer of ownership form and if they aren't going to accompany you to the LTD, a signed power of attorney form. You also need a signed copy of their passport face page and current visa/visa exempt stamp showing they are legally in the country at the time you try to have the name in the book changed, even if they have already left, and of course the bike and greenbook. Older bikes will need an inspection from an authorized inspection station. You will also need either a work permit or a letter of residency from immigration or your embassy.

Here are the two forms you need:

http://www.thaivisa....attach_id=62378

http://www.thaivisa....attach_id=62379

In English. (Not legal..use as guide)

http://www.thaivisa....attach_id=77494

http://www.thaivisa....attach_id=77495

^^ What he said, and I can't over emphasize the sensibility of going to the LTD/DLT? with the seller.

Get the transfer completed before the seller leaves. Do not wait till they leave or your got a mountain of problems.