Is there a bus from Merzouga to Ouarzazate?

Yes — the Marrakech-bound Supratours bus stops in Ouarzazate, about 6 hours from Merzouga.

This makes a dunes-to-kasbahs onward journey easy without backtracking through the desert. From Ouarzazate you can continue to Marrakech on the same service, visit the Atlas film studios, or turn south to explore the Draa Valley’s palm groves and kasbahs.