You need to go down to Cotounu,this city is the biggest and largest port for cars in Benin Republic,but due to the language barrier, except you understand french,you will need to pay an interpreter. Also if you don't know anything about Cars it is very risky as you may endup buying at a high price or buying a faulty vehicle. Then you will need to get the paper documents and most importantly the Carte-due-park papers, Benin Republic authorities don't joke with this,this is what makes you able to drive in their city,then head to Nigerian customs which will take at least two days to clear the car.

With the information I have been gathering so far,I don't think it is even worth the stress.