It is not possible to determine the quality of a fish based on its price.
Just because somebody has a site that tempts you means little. Its all about REPUTATION.
I get almost all of my fish shipped in. But this means anything from having a trusted breeder far away sending fish to my picking up boxes at the transshipper soon after they have been cleared by fish and wildlife etc. at the airport. So the exact same species of fish might cost me $20 from the breeders and $4 if it hits the country soon after being removed from its natural habitat in the wild. The first fish should arrive in good shape and be what I expect. The latter may arrive in poor condition, some may die, will likely need treatment and will need to be quarantined for a few months before being sold or put into my permanent tanks.
I also breed some pricey rare fish which I sell and this includes shipping them.
There is a fair amount of learning to get to the point where you understand the ins and outs of buying fish online/shipped in, especially from complete strangers. Never forget that Its all about REPUTATION.
There are plenty of bad sellers out there. There are a few good ways to find the better sellers:
1. Join a local fish club, the folks there will be a gold mine for new hobbyists. These folks have been buying fish for many years abd they know the reputable sources.
2. Before buying from the more commercial online retailers, look around for feedback from folks who have bought from them. Fish sites are great for this.
3. Even better are species Specific sites. If you are a guppy fan, try poking around these places: http://ifga.org/http://guppy-online.com/wga/
4. I hate to send you here, but what the heck. Remember, even excellent feedback doesn't guarantee 100% how satisfied you might be http://aquabid.com/
(I almost never sell my fish at auction, and then only at live events, never online).