i've gotten several fish off the internet and they have alwys arrived in good shape. I recieved one female betta who came out of the bag with breeding stripes, rather than the stress stripes I might have understandably expected.
HOWEVER, you are taking a risk. You can't see the fish, so they MAY be sick or poorly kept, or even misrepresented. Also, if you aren't willing to pay for a decent shipper, then you risk putting the fish through the regular postal service where they will be in transport longer and likely treated less carefully. Shippers like FedEx and UPS are much more responsible (in the US, don't know the options outside this country).
But it has its advantages. If you want a fish you cannot get in your area, the internet is about your only option. If the fish in question, however, CAN be purchased in your area from a reliable dealer (ie: good health and not misrepresented, which should be less of an issue if you research what you are purchasing) than you are better off doing so.