Actually, I think he just likes deals -- instinctively. He doesn't have the ability to make them magic here, since that power got taken away years ago, but I suspect he likes the transactional nature of it. He trusts it more than something more vague, like friendship or good will.

Also, he can't just possess someone who makes any old deal with him. The deal has to be worded in a way that implies possession is part of the deal. The agreement to work on the project while I was asleep meant he could only take over my body while I was sleeping. In my nephew's case, Bill asked for a "puppet" in exchange for a password to my old computer. The puppet he was referring to...turned out to be my nephew.