As has been said already... The LDS is a business, with a very, very limited sales base. They
absolutely depend on you to purchase from them. Some owners understand that they can't be pushy to get your business. Those are the ones you want to keep as a 'friend'. Buy what you can from them. In the long run, you'll wind up becoming a bigger part of thier community. There are others however, that will do anything to make a sale. Lie, beg, conjole, or threaten. Anyway possible to ensure that their trade remains intact. They prey on the new divers fresh in a dive course. These are the ones you want to stay the heck away from.
There are advantages to buying certain things from your LDS, over an online store. There are definately cost savings though buying online for certain items.
Suits, BCDs, Regulators. More often than not, 'most' LDSs will have suits and BCDs available at internet prices. If not, there usually isn't much of a difference between them and an online store. There are 3 reasons why you 'should' buy your suit & BCD at an LDS. 1. They'll fit it for you. Don't be a prick and have the LDS fit a suit for you and then turn around and buy it online for $20 cheaper. 2. You'll save on shipping costs. 3. If there is ever a problem with it, a responsible LDS will bend over backwards to fix, repair, or replace it for you, promptly.
Regulators & Octos. Almost all resellers sell Regs at the Minimum Advertised Pricing, including online stores. If an online store is selling a reg at substantially less than the MAP, then either its a demo unit, discontinued, or used. Either way, you wouldn't want it anyway, since it won't have a warrenty assosciated with it. The benefits to buying a Reg local, is that your LDS will know its history if you keep its upkeep regular. And, if you ever have problems with it later, they'll take care of you, usually.
Heavier items like weights & tanks are best bought locally as well.
There are a crapload of little things that may be in your best interest to buy online to save $$, like slates, lights, knives, mask & snorkel, fins, boots, gloves, etc. You'll find a better deal most of the time on those items.
Unfortunately, most dive classes require you to have boots, gloves, fins, and mask before the class and typically pressure you to buy on the spot when you sign up for the class. Thus your $175 PADI certificate winds up costing you around $600. You 'could' shave off a $150 or so off that cost. What'll likely happen though, is the instructor will frown upon you coming in with gear that he didn't sell you. Then you're left holding the bag trying to make up some kind of story about how you borrowed it from a friend or something.
Ackward....
At any rate... Shop around both your LDS and online. Just don't alienate yourself with your LDS. They will ultimately be your key to having fun in the hobby. If your LDS owner is a complete arse, find another one. If they're the only game in town... Sorry