I would recommend to sell the car by yourself by posting ads on Qatar Living or dedicated car selling websites like qatarsale.com , carsemsar.com . By going to the used car dealer shops you would be loosing some amount of money. Could be quite substantial depending on what showroom or what car it is.
I had tried Oasis but like most showrooms,if you want to sell your call to them they will reduce the price quite a lot.