carnet de passage
 

Hi there.
We are a group of 4 people wanting to travel with two cars through morocco, western sahara, mauretania to senegal/the Gambia.
Everything is pretty much sorted but the original aim of leaving the two cars we are travelling with in senegal and flying back home is in danger because of the carnet the passage business. What exactly is a carnet de passage and do we really need it for mauretania and senegal? Is it at all possible to sell/leave cars in Senegal/mauretania? What happens if a car breaks down and it would be impossible to take it back to Europe or so? How will customs deal with that. We are a fair bit confused...

Any help from experienced travellers concerning this issue would be very much appreciated. Thanks guys!
mig