I think comparing online clothing retailers to firearms retailers is a bit of an apples to oranges comparison. Everyone needs clothing, firearms not so much... Because of this clothing retailers have a much larger client base to draw from, and thus there's more money pumped into their sites. Firearms are a niche good, so the base they draw from is much smaller. Many other niche goods have effectively been absorbed into the Amazon monolith. If Amazon (or equivalent) were to not exist, many of these goods would have their own small niche retailers. These small retailers would have minimal capital, cue PHP hell.
One fascinating niche are online retailers which fill market voids Amazon deliberately creates out of corporate policy. For instance in the US one can readily purchase firearms and ammunition through the internet. Due to various legal formalities, Amazon won't touch that particular market segment. Visiting sites like www.sgammo.com, www.palmettostatearmory.com, or www.midwayusa.com, is like stepping into an uncanny PHP laden alternate reality where Amazon never existed. This federation of specialized retailers offers a glimpse into what the online shopping experience _could_ have become, if it weren't for Amazon's hegemony.