What if he bought him as a body slave, something vague about status or something, but chose him because he had the qualifications to be useful in the assassination/fighting department. Maybe Bucky's work will mean that Clint has to come with him places so he'd need to be able to play the part of good body slave but also be ready for other stuff if Bucky asked him to. Though maybe the latter isn't typical and could get them in trouble?