The internet provides unsurpassed opportunities for
buying fish, though it likewise means that the inevitable sharks are not necessarily of the marine variety! Due consideration of the quality of the website can often, though not necessarily, give a good indication of the standard of their wares. Paying via credit card, PayPal, Egold or similar companies ensures that one has recourse to the law, and you're unlikely to be bilked. Likewise, if one buys from an eBay or similar associate company, one can normally be sure of quality control, though you might pay proportionately more, but as with all purchases remember, Caveat Emptor - let the buyer beware.
Using standard internet search engines like those of Yahoo, Google, or Alta Vista will give you a plethora of choice as regards variety of fish, products, and price-range, but it may be best to use a non-partisan, 'cross-platform' search engine such as WebFerret. To avoid consuming vast amounts of browsing time, it's often worthwhile visiting a dedicated bulletin board, or forum to learn from others' experience, and to gain valuable hints, tips, and guidelines as to suppliers, product-lines, and quality. Above all, don't rush into anything, do your homework if you want complete satisfaction.
Let's look at a comparison between going to your local store and shopping online. At your local store, you can see what you'll get, the online brigade show you fancy pix, but they're unlikely to be the fish you actually buy. Locally, you can see the conditions under which the fish are kept, but you've no idea online. Local stores are unlikely to have the expertise, unlike some of the dedicated onliners. Some online sellers actually use expertise and long-establishment as selling points, though it's difficult to tell what's hype, and what isn't.
Cost-wise, you can browse online to do a price comparison; your local store's probably going to be marginally more expensive, especially if they have few competitors in town. Also stock-wise, the onliners are likely to have a wider variety, and a higher turnover.
As far as reputation goes, the onliners have a much wider customer-base, who won't hesitate to broadcast to all, and sundry if they're being cheated. Locally, you can easily find other
fish hobbyists to consult, but if you live in a small town you might have to travel to the city for satisfaction.
Travel, and shipment is a very gray area. You should only buy from an onliner that guarantees to replace dead fish. The further from you they are, the more problematic. You have to remember these are live animals we're talking about, they don't travel well. Temperature, change of climate, inconsiderate handling, all these adversely affect the fish.
Shipment is a cost factor to be wary about. The big onliners give you a choice of FedEx, Airborne Express, United Parcel Service and The United States Postal Service. Do you live near an airport? If so, collect the shipment yourself.
Finally, check everything out first by email or phone. You get a pretty good idea from their response what their products are going to be like. Would I buy online? Yes, but due diligence is the answer!
