I have had success receiving fish shipped in the mail. I don't know how they live that long like that, but I have had successful shipments from retail shippers as well as from the home hobbyist, and the only problem I had was when the hobbyist shipped them to me with too many tiny fish in one bag. She shipped replacements and separated them, and there were no losses.
As for the original question, I also like to watch fish at the LFS
for a littel bit before I buy them. I definitely think there is something to be gained by letting the shop hold onto them so any problems will become evident before I buy them. This is even more of a recommendation when it comes to otocinclus, because there will often be a rather dramatic die-off at the LFS
in the first 5 days after they receive them, so I wait and get the ones that have survived the initial week or so. Great question!