Its actually quite easy. Of course your local dive shop is going to hope you support them just the same as your local grocery store, home improvement store or even gas station are going to hope you frequent them.
Where they start to get wrong is by lumping all internet retailers together. There are retailers online who are not authorized dealers of the gear they offer you. What this means to you is customer service and support. They cannot offer you the full support of the manufacturer. If the item you are purchasing comes normally with a lifetime warranty you are sacrificing the ability to get the item serviced or repaired under the terms of that warranty. This can be a big deal when you think about the annual service on a piece of gear. Take that to another extreme and look at the recall page on the Scuba.com site. Every manufacturer of scuba equipment has had some item or other recalled in the past 15 years. If you haven't purchased from an authorized dealer you have sacrificed your ability to get any repairs, alterations or upgrades for free under the terms of the warranty.
And then there is knowledge. We are proud of the fact that the LOWEST certification level you will ever call and discuss equipement with Scuba.com is an Instructor. On our active staff you have 3 Course Directors, a Master Scuba Diver Trainer and a Master Instructor and an Instructor Trainer. There are certifications from NAUI, PADI, SSI and TDI/SDI. Among us there is a Biologist, a commercial diver, a travel agent, former manufacturer's reps, and the former Technical Support Director for several manufacturers. We are from everywhere. Hawaii, Florida, the Mid-West, Caribbean, the Pacific North West, New England, and of course California with exprience in most of the rest of the world.
What does all that mean? Information. If you want help with a piece of gear, a destination, an idea, or an instructional path somebody here has your answers. I don't know of many dive shops anywhere that I've visited in the world who can claim all that. And yes, I've visited BOTH of the retailers who like to consider themselves our competition.