Can I sue if they repo my car and sell it the same day?

As with all legal questions, you should seek the advice of a local attorney.

That said ... in most places the lender is required to give "reasonable notice" to the borrower that the repossessed car is to be sold, and generally "reasonable notice" is considerably more than "a few hours". It could definitely be worth your time to consult a lawyer, especially if the lender sold the car for less than you owed on it.