Returning the fish would be the best thing to do. I wouldn't go to an LFS that would allow fish to be sold with a non cycled, way too small tank.
If they won't return the fish, your live rock would definitely help, also give them some of your sand too because that has the bacteria. Do you have any other filters running on your tank with media or sponges? Any surfaces in your tank will have bacteria that they will need, so give them what they can.
If they won't return the fish, your live rock would definitely help, also give them some of your sand too because that has the bacteria. Do you have any other filters running on your tank with media or sponges? Any surfaces in your tank will have bacteria that they will need, so give them what they can.