I think about this a lot. I teach Marketing at the local college and one of my preachings is, "if you buy them on price, you are going to lose them on price." But yet I search the internet a lot trying to find the best deal. I believe strongly that the value of the internet is the wide area of communication.
I also teach that price is a function of four things: cost, time, quality and risk.
If you get it right away, it costs more. The more risky the purchase (such as ebay), the lower the price to compensate for the possbility of a bad purchase.
After being burned on ebay, and seeing lots of copied pictures on ebay, I limit my purchases to only low valued (under $100). As a rule of thumb, if I don't save 30% on ebay, I go either internet store or LBS.
I have also found that the Internet Stores on smaller purchases have little or no savings over the local LBS due to the shipping/handling charges. One way the Internet Stores have lower prices, is by selling their own branded merchandise (private label, such as Nashbar). However, these private label items are often poor quality (my Nashbar pedals had the bearings wear in a year and they could not be replaced nor lubed, zipper on the seat bag broke in a year).
Bottom line is that I save my on-line purchase for a couple of larger orders a year (such as tires, tubes, chains, lube, etc) and go to the LBS about every two weeks for the smaller purchases the rest of the time. I would say that
40% of my purchases are online and 60% at the LBS.
I think that is fair to the LBS, but also allows me to save some money. It is through competition that the businesses become more efficient.