Sounds to me the cashier was just feelingless and rude, if I were you I would try not to pay much attention to it or perhaps just respond with something positive like "Well I hope you have a fantastic day too" next time. This will put the ball back into his court and perhaps make him put more thought into what he says to others.

In the end, what does it matter what this person you do not know thinks or feels about you, they are his emmotions and he will have to deal with them, just try not to allow them to have a negative effect on your emmotions or life.