I think the CL seller is being completely honest in their responses. Less than 1% of the people on this forum could correctly sex day old chicks. It takes most of us one month or more to get it right. Hatcheries can correctly identify sex in hatchlings about 90% of the time and they are doing 1000s at a time. Until the chicks start developing identifiable differences between the sexes, you are just relying on your gut and that's not 100% accurate even after raising 1000s of chicks. This person is taking the risk of getting roos out of the equation by growing them out a bit and eliminating the obvious boys. Chickens do die for no apparent reason and nothing you did or could have done differently was going to change that. Most chicks that are going to die do so in the first week to two weeks. If you are buying them after 3-4 weeks they are more vigorous and less apt to have problems. Any developmental problem has also been identified and culled so you don't have to do that part. It is part of owning chickens but can be traumatic to children.
This individual has a very nice selection of chicks. I would suspect that there are from a hatchery just because no one individual could raise and maintain that many breeding pens.
I buy and raise about 100 hatchery pullet chicks a year and sell them at 3-4 months of age. I have a great market for pullets that are close to laying and who have had most of the risk of raising them removed. Most of my buyers live in towns that have a rooster ban and to date I have had 6% roosters. I do choose breeds that are more family friendly and can be tamed with little work. I had many repeat buyers and referrals. I would have no problem checking out this seller. If they have a problem with you coming over and walking through the pens, that is NOT a problem. They are protecting their birds.
buying from feedstores has it's risks as well. Most of the employees are uneducated about chickens. You could easily end up with different breeds than you through you were buying and the stores typically will not guarantee their chicks as pullets. If you get a roo, it's your problem.