There has been an explosion in the number of products available in the marketplace. No local retailer can possibly stock everything available--it applies to more than just firearms.
A majority of online firearm offerings now seem to be from stores with a physical location, and some are very large operations with aggressive pricing. More stores are using the Internet for at least some additional sales. Pure Internet-based retailers are often small operations, one person, which operate very efficiently as they don't have heritage (expensive) locations and procedures. The same person can now take care of advertising, accounting, purchasing, shipping, web design and more--so costs are lower.
Main-street retailers who favor MAP and Internet Sales Tax are hoping they'll recreate the status quo. They won't. More efficient companies, large and small, will chip away at their market share. Big-box and Internet selling is fiercely competitive, which drives down prices. There's almost always a lower price available for anything as some store is going out of business, got a free gun from the manufacturer or distributor or is running a loss leader. Main-street retailers would be better off establishing their own online presence, finding an under-served niche, or specializing in unique items.
I have a retail store open six days a week, in a rural area, on an island, selling shotguns for sporting clays shooters. My guns start around $1,500 and go to around $10,000. I maintain an inventory of about $250,000 of these shotguns. I stock shotguns for lefthanded customers, youth, women and even left-handed women shooters. People can't get these guns in many parts of the country without the Internet. Websites excel in finding rare and unusual...