To address the OP question, one needs to be careful when buying on-line. The most common problem is photoshopping frags to appear better than they actually are. Even if the pictures have been white balanced, your computer monitor may not display them accurately. Generally, if it looks too pretty.. well.. buyer beware.
I have used LiveAquaria, Kevin Kohen's on-line marketplace, for over 16 years with excellent results. Their shipping is top notch and free for purchases over $150. They also offer a 14 day, no questions asked guarantee. Why would I purchase from LiveAquaria? Often times the animals that I am after are quite hard to come by.... and Kevin is known for getting in the unique. Even after the acquisition of Fosters & Smith (owners of LiveAquaria) by Petco, the service and quality remains outstanding.
Last year I purchase an adult Maze angelfish from LiveAquaria, an animal a bit north of four figures. As with many Cheatodontoplus sp, it decided to pout and not eat when it arrived. I contacted Kevin about the animal looking great, but refusing to eat after two weeks and he guaranteed the fish till it ate, regardless of the amount of time that took. My Tinker's butterfly pair came from LiveAquaria, as did my Gem. Several smaller fish and corals as well...
Their QT and supply chain are second to none....
On the other hand, if I was looking to buy a yellow tang and some frags... well... buying local certainly makes sense. And yes.. I own a yellow tang who happily lives in a Nuvo... Spaz (my wife named the yellow tang) came from Cris at Aquatic Art. Most of my corals come from Cris... though I have some nice pieces from Josh (Gonzo), Jeff (Aquamart) and Reefkoi as well.
If you are looking for corals, clean up crews and more common fish, our local vendors have great selections. If you are looking for something a bit more esoteric, online may offer some good alternatives.