Every time I’ve sold a beat, I’ve given the buyer the separated tracks of each beat, which is standard practice as far as I know. It’s not really to prove I made it, it’s for mixing purposes. But I probably wouldn’t have the separate elements of a beat unless I made it, so I guess that does prove its authenticity.
Although it never came up, I wouldn’t have a problem signing something that placed liability on me, except for my sample beats of course.