Seems like the goalposts have moved (again) so you must now provide your Turkish ID Number to sell a vehicle.
No problem if you have a residency permit though...
http://tckimlik.nvi.gov.tr/Web/ForeignQueryIdentityNumber.aspx1. Enter Resim Dogrulama - The graphic number on the right.
2. Dogum Yili - Your year of birth.
3. ikamet Tezkere No - The number in the front of your book xx/xxxxx.
4. Sanal klavyeyi kullanmak istiyor - Check this box.
5. Then hit Return to get your ID number.
Hot Tip: Now write your ID number on the front page of your residency permit.
You are now officially just a number!

Mike
