You can't vet an online retailer that isn't even in your state, they can have the nicest website and their fish keeping is tick poor.
You absolutely can vet online retailers, whether of fish or anything else. You ask people who have purchased from them and get some rough statistics on how many bad experiences there are out there. You look into their business model in general -- low price leader or aiming for high quality? Run by anonymous people who don't care, or by someone in the hobby who you've seen and even talked to here and other online venues? And so on.
I think your missing the point (or own a lack luster shop)
I don't own an LFS, and I'm not missing the point -- I'm tracking changes I've seen over decades. Over the last 30 years I've been in it, the hobby has changed, a lot. The decline of local independent shops is nothing new, and is the entirely unexpected effect of a massive market shift that started with Dr Foster and Smith in the mid 1980s (remember the mailed catalogues?), and to some extent the crown prince of the race to the bottom, PetSmart. There's really no way local shops with all the financial outlay that entails can compete with the market that now gives me access to every single online retailer. A local shop is competing with scores of online vendors that don't have to have a nice storefront with customer restrooms and clean floors and displays of every food and additive anyone could want and employees that not only know their stuff but have some minimal customer service skills and open seven days a week. A local shop isn't competing with all the other local shops, it is competing with every online retailer in the US. And all those ship right to my door, for like $30, and I can shop at home with a beverage in hand at midnight.
A person might as well suggest that the reason that almost all the malls went under is because they had "poor ownership". That shift (away from brick and mortar local clothing, accessories and electronics shops) was the effect of online retailers. Was Blockbuster poorly run, or did Netflix figure out online video distribution? Did local newspapers all suck, or did online news distribution simply push them out? Were the incredible multitude of various mom and pop stores all poorly run, or did Walmart and then
Amazon beat them into submission? Why were all these poorly run stores doing just fine until the technology was mature enough for the online shopping revolution? That those two things are just a concidence would be unbelievable on the order of 'must have been space aliens'.
The sorts of shops you're likely thinking of have been tried. Saltwater Empire in Minneapolis was like this; they lasted, what, five years before it just wasn't worth the effort. There were a couple in Chicago that I don't recall the names of, same deal. They were absolutely not poorly run shops. There just isn't the right market structure to make doing it right a reliably profitable venture.
