Were deserts once oceans/seas? I was reading _The Alchemist_ and stumbled upon something that says, !The Alchemist, Page 104 > Here and there, he found a shell, and realized that the desert, in remote times, had been a sea. Were the deserts once oceans/seas?

Yes, some of them clearly were, for example whale fossils have been found in the Sahara at the Wadi al-Hitan. However, while some deserts were once oceans/seas, that doesn't mean all deserts were once oceans.