Welcome on the Hubb,
Your questions are difficult to answer since they are very unspecific. Nevertheless I am going to try to give you some answers.
In several West African Countries it is quite easy to sell you car, since they do not require a carnet and do not register the vehicle in your passport (Mali, Niger, Burkina) therefore as a seller you do not pay any taxes.
In other countries (Mauritania) a fee for deleting the car from your passport is payable before you leave country.
I personally sold a car in Gambia and found it quite easy , since the taxes are paid by the buyer when he registers the vehicle.
I suggest you read through the recent post on the issue of selling a car in W Africa on the HUBB and buy a decent Guide book like Sahara Overland.
After that you will be able to ask more specific questions, that will be happily answered.