When you consider that a single merchant has the potential to make over 1000 florins a turn standing on one of those gold or ivory resources, I'd say yes. You can't reach Timbuktu by sea anyway; it's a landlocked province. Arguin is the seacoast one, and yes, it is unreachable by sea until after oceangoing ships are developed in the 1400s. You have to march overland. Go east from Marrakesh towards Algiers. At the mouth of the river turn south and head through the pass. Once across the mountains bear back west for a short distance and then turn due south across the Sahara. Timbuktu is way down there. It takes about 8-9 turns of walking from the coastline to the city, plus another turn or two to the nearest resource.

Once someone has taken the province, it's possible to build a road there that shortens the traveling time a little. The Moors will get it at some point, though there's no telling if they'll manage to keep it. In my last game it revolted on them.

Side note: Sahara is Arabic for desert, so wouldn't the name "Sahara Desert" be a multi-lingual redundancy? Desert desert?