I know time can't be it, since it seems to me that a person could ride for years and years and yet never be 'experienced' or conversely, a new rider with appropriate training and dedication could become an 'experienced' rider within a few years. It seems like it would have to be the skills and abilities that define the rider.
Do you think the definitions are pretty well known? For example, if seller A says that the horse needs an intermediate rider, do you think most other people would agree with that? Or do people vary a lot in what they consider to be beginner/intermed/adv?