Hello, 

I would like to know which documents I have to supply or sign in order to sell my car, copy of passport for sure but what else? Is there a page to fill in and sign in the blue book? Is there a special document to get from the Transport Office before I leave the country and ask my friend to sell my car on my behalf?

Thank you

You don't say where you live, but I just sold my truck in Chiang Mai.  I suggest going to local Transport office and ask.  I needed the transfer forms, a "Power of Attorney" form, signed by myself and two witnesses.  This authorizes your friend to act on your behalf.  A copy of passport and visa. Also a Residence Certificate. This could be a problem for you as they are only valid for thirty days.  Make sure your passport number matches that shown on your blue book.  I had changed passports during the life of my vehicle and they wanted to see a copy of both passports.  Bottom line: Before leaving, Check with the transport office your friend will use.