There were only four worthwhile and practical things to trade to the Novans to keep the fleet running. Technology was one, but it was under ban by the Council and had been for centuries. Besides, what Earth had wasn't all that far ahead of what the Novans were capable of making for themselves now. Gold? Half the gold of Earth was already on Terra Nova; same for the silver, platinum, palladium and rhodium. There were plenty of proles to trade as slaves, but the Novans, most of them, had little use for slaves. And the Moslems, and especially the Salafis, who did have use for slaves, only wanted pretty young girls and boys. Since there was a strong market for those on Earth as well, saleable slaves were a tight commodity. Moreover, you never really could tell what the proles knew. If they were questioned, and the Novans realized what Earth had become, it could be a disaster for the fleet as well as the Earth.
Art, Robinson sighed. I am reduced to selling Earth's artistic patrimony to keep in being the fleet that keeps Earth from being overrun in a hundred years or less and looted of, among other things, its art.
Ah well, I should be grateful I was able to talk the caliph into turning over to me so much of the contents of the Vatican's cellars. Fortunate, too, that he valued them so little. Then again, with even the followers of Islam so few, and most of those barbarians in the reverted areas back home who could care less about the caliph, I suppose he needed the credit as well.