IMO I actually trust online fish retailers a bit more than a LFS and certainly more than big box stores, here's why.
A LFS sells tanks, filters, other equipment and supplies. That makes up the bulk of their sales and the actual fish themselves are a small part of sales with not a lot of margin built in.
An online fish seller ONLY sells fish. If that's all they do they are much more likely to really take care of those fish including quarantine new arrivals. Their entire business is built upon delivering good quality fish and gaining a good reputation for doing so. This is a niche hobby and word travels fast if a vendor consistently delivers dead and or unhealthy fish. They won't stay in business for very long.
Now look who shops at these places. A brick and mortar LFS has people walk in off the street. New hobbyists, or people that have never kept fish before or some peeps that just have their one betta or some tetras and that as far as they go. Online vendors are specialty shops that really cater to more advanced hobbyists like ourselves and others that would actually take the time to participate in online forums. Probably like 10%, maybe 20% of the hobby.
If you asked the newb who just purchased their tank, they probably have no idea you can buy fish online and have them shipped to you.
There is value in seeing the fish with your own eyes and seeing they appear healthy, on that I agree. Therr is also value in joining a local fish club. There are always auctions, swaps, trades going on at fish clubs. Fish grown by dedicated hobbyists. That's honestly where your best fish are going to come from.