I am going to buy a motorbike (OK scooter) for my wife. I have always bought new and let the shop handle the paperwork (with the exception of certificate of residence). What do I need for my wife to buy a secondhand  bike and how do I go about it.

As far as I read about it, you need her ID card (or copy) and a proof of address, now that comes in variety of forms, depending on if she owns the property or rents - I think all Thais know what this is and how to get it.... 

Let the shop handle the transfer.

1 hour ago, Agusts said:

As far as I read about it, you need her ID card (or copy) and a proof of address, now that comes in variety of forms, depending on if she owns the property or rents - I think all Thais know what this is and how to get it.... 

This ^^^

It's quite easy for a Thai to register a bike.