It is hugely unusual to see anyone reputable offering kittens for homes that are only a couple of weeks old. Breeders should be interested in selecting which kittens they keep back for future breeding on the basis of best possible temperament, type, and colour/markings.
When you are looking at kittens that are very young, you know what variety they have and what markings, but you cannot properly evaluate temperament or type. So that means you are only selecting on what variety they are – not even the quality of it, since the colour and markings can change rapidly in the next few weeks.
If you do have an interest in breeding to improve temperament/type/colour, you should make decisions on the kittens you are keeping and those you are not as late as possible. This would allow you more time to properly evaluate the kittens, and it’s also better for those purchasing kittens, as you can tell them more about the personality of the rats involved rather than it just being about what they look like.
By properly selecting the rats you keep as potential breeders in the first place, you’ll be able to breed to improve, rather than breed just to create more kittens of a particular variety.