Sorry, but I disagree. I'm sure there would have been caravans across North Africa selling goods of all sorts, including slaves. The Sahara was, as has been pointed out, savannah as well as desert, and not as inhospitable as today. As for camels, Caesar introduced them to Africa in the first century BC. Camel bones have been found in Ostia and there is a VIVARIUM or animal holding area just south of Ostia (for beasts brough from North Africa for the arena in Rome). There is, I believe, a Roman toy camel in the British Museum.
Also my characters travel by camel caravan from Sabratha to Volubilis in my forthcoming book... so that proves it!
Flavia