Let’s look to the stock market as an analogy. Is it unethical to buy stock in multiple companies with the intention of making a short term profit? Will it be unethical for baby boomers to pull their retirement funds out of stocks, cash in their profits and live off the proceeds? Is it unethical for you to put $$ in a education stock fund for your kids, with the hope of making good returns and sending them to college?
Definition of ethical = Conforming to accepted standards of social or professional behavior.
Flipping still passes the professional test as it is not illigal and is actually promoted by corporations and government policies. Whether it conforms to socially accepted standards is the real question!