There are many people who call themselves breeders. Selling babies does not make one a professional, however.
Professional breeders invest in the health of their birds, select the parents based on specific qualities, keep track of babies hatch dates, treat babies as individuals, keep records, provide toys, vet care and socialize their babies. In the United States the professional standard is also to only sell unweaned babies to experienced hand feeders that one might also deem to be professional or to raise them to the point of weaning.
Sadly there are many breeders who are not professional. That doesn't mean that you will not get a good baby but unfortunately there are things you may never know about the bird. Also if the breeder does not keep records of hatch dates I doubt they keep other records which could be invaluable to them in improving the health and care of their flocks.
As Zara mentioned, some babies do grow more slowly. One common cause is if they did not receive as much food the first few days (this can happen with poor handfeeding or even when a parent bird does not provide abundant food) to compensate for the less abundant nutrients the babies have a slowed growth rate. This is good because it allows them to grow with the nutrients that they are receiving rather than having a fast rate of growth and not having nutrients to support it. So you get a healthy baby but just one who grows more slowly. They can mature weeks later than their clutchmates but as I said will generally be just fine.
You should be mindful that you are feeding your baby often enough as scruffy feathers often indcates that feedings are too far apart, the wrong temp or that the bird has not been keep warm enough.