yep , in Mauritania your passport get *ussually* stamped with your vehicle's details.
If you sell your car over there , then you should go -with the buyer- to the douanne's offices to "clear" your passport . This is a common procedure and there are fixed fees on that . I think is on the region of 200 Euros .
This is normally payed by the buyer ,althought -being Mauritanians they way they are- is not uncommon that -after he has the car- refuse to pay you the clearing fee , flee away , and then you have to go on your own to pay the full ammount -200 Euros -.
I would advice not to believe the buyer's offer to clear the passport , do it by yourself , and just add those 200 euros to the final price.
Have a nice day.