I have been watching this thread for a couple of days, trying to decide if/when I might have something to contribute. And wow! there is a lot to say on the topic. Full disclosure - I own a small LDS, close to a one man show.
Brands carried: There are a lot of variables here. There are some brands that limit the number of doors they will open in a given area, while some other brands will open anyone. And often the decision is up to the regional sales rep and not a centralized authority - so even that can be inconsistent across the country. Yes, there have been brands I asked for, but was turned down due to proximity. (main factor - don't piss off an established account by opening another account right next to them) Then there are buying levels. If I was able to put all my eggs in one basket, buy all categories of goods (Regs, BCDs, computers, wetsuits, masks, fins, etc.) from only one vendor and never lose a sale, I would have a much better buying level and better margins versus splitting my inventory across multiple brands. So even on brands that I can open an account, inventory investment is a balancing act between choice, selection, and best cost.
LDS On Line: There are a lot of variables there, too. Aside from MAP pricing, most vendors have an extensive and varying set of rules regarding what and how a dealer can sell on line without violating their dealer agreement. Without going into detail, in can be onerous and complex - requiring resources and back end commitment easily beyond the reach of a small business. Worse yet is biting off more than you can chew, and creating the opportunity for negative customer experiences. Don't do it if you don't think you can do it right. I don't have industry and market share numbers, but this is a relatively small industry - that pie is only so big. Just how many successful, similar scale (and therefore similar pricing) large online operations can there be before the market is saturated? If all the sales were done by a (relatively) few online LDS, I suspect the majority of local shops would disappear. Which means goodbye to the primary source of training, service, airfills, etc. that enable your local diving.
LDS buying online from cheaper than vendor sites: That is a nonstarter. I would not be able to offer the manufacturer warranty on anything bought that way. And although it might not be likely, it would increase the possibility of my acquiring grey market and questionable product.
Product Selection: Again, the smaller the scale of the business, the less able to provide depth of a product line. Just think about what it takes to stock a single wetsuit in 3 thicknesses, 2 genders, and lord knows many sizes. And then add colors, and maybe another brand or two?!? Now, how many BCD models in how many sizes and how many brands? It adds up to a big number REALLY fast.
So yes, I am between a rock and a hard place, trying to sell equipment for a fair price, while knowing that I can't possibly meet and/or beat every online price. There can be some crazy stuff out there. I try to provide a reasonable choice, bring in items that cover as many "customer solutions" as I can and cover the core needs and wants, while being responsive to ordering other items as needed. And help the customer make the best choice that best meets their needs.