An OBD reader is a tool that can communicate with your vehicle and highlight any problems that are present. Some tools provide the facility to clear fault codes while some do not. If you take your car to a garage offering diagnostic services they will be able to clear the codes for you, but that does not guarantee that they will not return if the fault is still present.

If you want the best possible price when selling your vehicle it would be worth investigating the cause of the codes yourself or taking it to a garage and ensuring that the components responsible are replaced or repaired.