He did not buy it properly then. When you buy a second hand vehicle (motorbike or car) both party's have to go to a notaires office to legally transfer it over(the buyer foots the bill for this) Once there, the seller has to produce a certificate from the tax office that there are no outstanding fines etc on the vehicle (if there were the seller would have to pay them to get the certificate)
Without this certificate the vehicle cannot be legally transferred. The cost of re-registering from a Turkish plate to a foreigners plate is complicated and it is best to employ a takobci (probably spelled it wrong) to do this, all Notaires offices have one nearby. As always, do things properly and you will have no problems, try to do it on the cheap and it will cost you a lot of money. No matter what the seller tells you, the above is how to do it legally and without any problems.