It is not ethical to buy a degree. When a university grants a degree, it says that the recipient has done a body of work, learned a broad set of data or information, and thus earned the degree in question.
To simply buy a degree, or for a university to sell a degree, is by definition unethical, with the exception of certain honorary doctorates. Governments worldwide are taking measures to shut down universities that grant such diplomas, and fraudulent degrees have led to the dismissal of quite a few well-known figures, as reported by USNews.com. Beware of schools that make it too easy to get a degree.