Since the question is a year old so will probably not be usefull for the OP, but the thoughts and specific information about the last portion of the journey may be of interest to others.
Your idea on getting information on possible Convoys should be priority Number 1.
Based only on the collected information about Mauritania, travelling outside a convoy would not be advised.
Arrangements must be made on how the used car is to be dispose of beforhand.
Based on the UK travel advice, the border procedures alone can take hours.
Add to that the Customs procedures for importing a car to be sold with any needed negotiations in one of the official languages (Arabic and French) may make this an impossible task without any prearrangements.
Using the car with on any trip on the Iron Ore train might be a good idea, even if you get off at Choum. Getting back to Morocco (through the West Sahara) doesn't seem possible and if, risky since it is a disputed area.
Entry requirements - Mauritania travel advice - GOV.UK
You need a visa to enter Mauritania. This is obtained on arrival. The current cost is €55 (euros) or $60 (US dollars) in cash only. The visa issuing process at the border can take several hours. Visas can no longer be obtained from the Mauritanian Embassy in London.
Regulations and customs in Mauritania : Customs
- from Groupe Société Générale (not an official Mauritania Customs source)
Customs Duties and Taxes on Imports
20%, Mauritania remains a country with high custom duty tariffs in comparison with its neighbors.
The evaluation of the value of products is based on transactional value, except for used cars, where the minimal importation value continues to be used.
What this actually means is a good question.
Assume either the going rate of buying such a car inside Mauritania or a satisfactory bribe.
Iron Ore train
Mauritania Railway – Choum, Mauritania - Atlas Obscura
- no date given in the blog article
on its journey across the Sahara Desert from Nouadhibou, on the coast, to Zouérat, an iron mine in the center of Mauritania.
But iron isn’t the only thing this beast of a train typically carries. Additional passenger carriages can be attached to the train, but most often, passengers sit on top of the ore. The train can carry road vehicles as well, and if you are self-driving, you have to make arrangements for your vehicle to tag along for the ride. Once your vehicle is on the train, you can decide to travel inside it, on top of the ore, or a combination of the two.
Hitchhiking on this train and sitting atop a bed of iron ore is not an easy ride, but it can be the train ride of a lifetime. The train runs through one of the harshest environments on earth. It’s sun-scorched during the day and bitterly cold at night, and you’d better prepare for both, as there is no shelter on the train, unless you brought your own vehicle to hunker down in. If you’re wondering about toilets, the answer is no: there aren’t any. There is also no way to stock up on your food supplies.
Only passengers with their own vehicle usually go all the way to Zouérat, as apart from the iron ore, there is little else there. Most people hop off (or get on) at Choum,