You don't have to look far to understand the title of Repossessed. Kiriel takes the body of a teenager who is about to die, figuring the guy won't miss the final sections of his life since he's pretty much road kill at this point—in other words, he repossesses Shaun.
Throughout the book, we hear Kiriel worry that he'll have to give the body back. It isn't his, after all. He tells his pet cat, "The repo men haven't come yet […] Get it? Repo-ssession?" (15.10). This adds another layer of repossession to the book: As much as Kiriel repossesses Shaun's body, he also worries about keeping it from being repossessed (re-repossessed?) by its original owner. Repossession, then, is both what launches Kiriel's adventures and, ultimately, what ends them, too.