I buy pretty well all my fish on line. All depends on the type of fish you are buying and who you buy them from.
Some fish just do not ship as well as others do.
I'm very impressed with the fish that LiveAquaria.com sells. I've made several purchases and have only had 1 shrimp DOA and one guppy "injured". Guppy made a full recovery. Yesterday, in fact, I received a school of 10 rummy nose tetras and 3 rasboras from them. The fishes' health was excellent. Not extremely stressed and they didn't even try to hide in the QT tank. They just want some food
I've purchased two lots of fish from ****************. I bought a couple of hillstream loaches which did not ship well at all. They were very pale when they got here, would not eat, and died within a week.
Just got another one in today -- 7 SAE, 2 mystery snails, and 6 amano shrimp. The fish are stressed, but look healthy and the shrimp shipped extremely well. They're in my other QT tank and the SAE are still rather jumpy and stressed looking. I've covered the tank with a large towel hoping that they will settle down a bit. I do not know if I will order from this place again.
I've bought several "rare" fish from various dealers on AquaBid and have had no problems at all. I'll continue to buy from them.
I've never bought a fish online that had a disease and that's probably because if they ship fish carrying ich or something worse they could lose their entire reputation. Word of mouth carries far on the internet.
You take your chances when you order on line, but I've less success from buying from a local chain shop (Petsmart, Petco). You never know if the fish are labeled properly or not, if they have ich (and many of them do) or some other parasite.
My local LFS has great fish, but not a great selection and definitely not the fish that *I* want.
Roan