its simple, you do not need dealer.

just get the cash car value from your buyer.

go to trafic police department (madinat khalifa or any other)

you will need your Qatari id, buyer Qatari id, and a letter from his sponsor NOC for car

you will find typest outside the traffic police office, they will type the form for you.

go to registration, and sign the form at front of police officer, they will transfer the car to buyer name

good luck