Secondly, you underestimate the power of local support. I'm definitely not waving the Geek Squad flag here, but with some purchases it's better to go with a local vendor for faster support.
A good friend of mine works for a local independent high-end A/V shop(maybe 15 employees total, including installers), and he runs in to this on a daily basis. Their main consumer base is composed of people who want to buy quality equipment from a sales staff that actually knows their shit, and go to a place where "customer service" doesn't mean beating somebody over the head with extended warranties. And from personal experience it seems most of these customers have at one point been burned or fucked-over by the Best Buys and the Circuit Cities of the world and just want to talk to an informed employee in a low pressure environment.
There is no rebuttal that will appease a customer that walks in the door with a printout from a fly-by-night online retailer or eBay seller quoting a receiver at 1/2 price off of a unit that normally retails for $2000. There is absolutely no way to compete with that from a pure price standpoint. What those people fail to recognize is that if something goes wrong...if the unit ships damaged...or if you need help in designing or setting up your system...THAT is where the local stores have a distinct advantage.
Personally, when it comes to buying things that I am 100% confident in my ability to fix or setup myself (computers, car parts, etc.), then it's time to fire up Firefox and break out the credit card. But if I am going to make a big purchase and I am still unsure about the advantages and disadvantages to certain products, or if I just want to see it in person and play with it, then it is pretty hard to beat a reputable (key word) B&M store.
I have a theory that it's impossible to prove anything, but I can't prove it.