The Time Traders by Andre Norton

Murdock lay on his back, gazing up at the laced hides which stretched to make the tent roofing. Having been battered just enough to feel all one aching bruise, Ross had lost interest in the future. Only the present mattered, and it was a dark one. He might have fought Ennar to a standstill, but in the eyes of the horsemen he had also been beaten, and he had not impressed them as he had hoped. That he still lived was a minor wonder, but he deduced that he continued to breathe only because they wanted to exchange him for the reward offered by the aliens from out of time, an unpleasant prospect to contemplate.

His wrists were lashed over his head to a peg driven deeply into the ground; his ankles were bound to another. He could turn his head from side to side, but any further movement was impossible. He ate only bits of food dropped into his mouth by a dirty-fingered slave, a cowed hunter captured from a tribe overwhelmed in the migration of the horsemen.