Young kittens, especially strays frequently are affected by several viruses that together are called the Feline Upper Respiratory Syndrome. If the infection is already in the recovery phase the sneezing may persist but the kitten is no longer feeling ill. Viruses are generally not treated with specific medications but rather with supportive care, which sometimes includes antibiotics for secondary bacterial infections.
A pet store is no place to pursue medical care, no more than a department store would be for a person. If in doubt about your cats health see a licensed veterinarian for diagnosis and treatment recommendations.
Please select and visit a veterinarian for your kitten. There is undoubtedy other issues to address, especially parasites such as worms, ear mites, ticks and others.
Good luck!
the cat was not examined by a vet before because it was a stray sorry that was a typo.