I've had very good luck ordering fish online. I have gone through: liveaquaria, aquacon, reefs2go and petsolutions. I found them all to be very good except for aquacon. Their guarantee is only for a credit, not a refund and their corals were horrible. :-( I have had really good luck with ordering fish though thru aquacon so go figure. lol
keep in mind if you are using a credit card to order that the card has protections just in case something goes awry you can always work with the card company if you get screwed over by the seller. Just an in case situation.
I do prefer to buy locally and be able to see the actual fish or coral but with the prices offered online it's hard not to take a chance and in my situation I have been happy 95% of the time pretty much.