A kitten should not be sold or given away until it's 12 weeks old. Buying a kitten younger than that could cause many problems.
The kitten should be inspected by a vet, microchipped and vaccinated at least 7 days before you bring it home.
The cat should be vaccinated when it is 8 weeks old and then again when it's 12 weeks old. If your cat is not vaccinated when you buy it, you should book a time to do so.
Talk with your breeder about the cat and ask if the cat has been socialized at an early stage.