What would happen if the country's largest online retailer swallowed the world's most popular trading marketplace? That's the scenario recently suggested by New York Times Bits blog editor Saul Hansell.

In arguing that Amazon.com (NASDAQ:AMZN) should acquire eBay (NASDAQ:EBAY), Hansell marries the notion of eBay's chunky margins with Amazon's penchant for ingenuity.

"For years it was impossible to even suggest that Amazon buy eBay because eBay's market value was three or four times that of Amazon," he starts, before rightfully pointing out how Amazon has nearly caught up in market cap to become the true market darling.

The reader responses -- now five dozen and counting -- have been mostly ruthless attacks on eBay. That doesn't surprise me. I write enough about eBay to know that a kind word about Meg Whitman's company is enough to fill my inbox with venom from packs of disillusioned former Power Sellers and cheated buyers on the trading site.

As a handshake enabler, no one ever expects eBay to be perfect. I'm sure there's a bummed-out buyer somewhere still fuming over a Snoopy Pez dispenser he never received 10 years ago.

Knocks on companies, particularly fast-growing companies, are usually swept under the rug. The problem with eBay is that it's no longer a fast-growing consumer-to-consumer trading site. Sure, PayPal is a star. eBay probably overpaid for Skype, but at least it's growing quickly. The stinker here is that eBay's own namesake site is struggling.

Auction listings at eBay.com have fallen, year-over-year, for two consecutive quarters. This past quarter treated marketplace bashers to the first sequential dip in active eBay.com users in its history.

It's not getting any better. American Technology analyst Tim Boyd is bullish on the stock with a $50 price target. He's tracking the current quarter and projecting healthy gains in international listings, but yet another year-over-year decline in domestic items being put on the block.

The rise and fall of eBay
Where did eBay.com lose its way? Greed has played a part. If there has been one ominous trend in eBay's quarterly reports over the past few years, it's that marketplace revenue is inching higher than the gross value of the merchandise successfully being sold on the website.

"I still believe that if eBay continues to skim more off of each transaction, it will continue to erode the site's value proposition," I wrote back in April, and that was before the stateside listings turned negative.

There are just too many outlets for moving discounted or homegrown goods these days. It may have launched a cottage industry, but now many of those cottages have filed change-of-address cards. Sellers have moved on. The heavy hitters are selling through Amazon. The mid-sized players are launching cost-effective pay-per-click campaigns through Google (NASDAQ:GOOG) and Yahoo! (NASDAQ:YHOO). The small fry are holding their garage sales virtually through Craigslist or Google Base.

Auctions may be exciting overseas. You see it in the success of MercadoLibre (NASDAQ:MELI) in South America and Gmarket (NASDAQ:GMKT) in South Korea. But they're just not cutting it here, where broadening selection of marked-down goods coupled with instant gratification is taking the air out of eBay even with its patent-bending "Buy it Now" buttons and whimsical positioning.

This is why eBay is worried, and rightfully so. Even though more people are online today than a year ago -- and, no doubt, trusting more in the site as a platform for commerce -- eBay's popularity is diminishing domestically.

So why would Amazon snap up eBay when it's the one doing all the right dance moves?

The fall and rise of Amazon.com
Accelerating sales growth? Year-round profitability? Are you sure we're talking about the same Amazon?

We are. Jeff Bezos' online empire has blossomed in recent years. It was always a popular haunt for shoppers, but now even cynics have come to eat crow under the e-tailing giant's financial statements. Did you know that cynics who pay $79 a year to be a part of Amazon's Prime program can have that crow delivered for free in two business days? Yum!

The problem is that Amazon may never shake its low-margin ways. It's definitely trying. Amazon is now selling digital versions of the same books, CDs, and DVDs that it began selling as an upstart in the 1990s. There are definitely better margins to be had in digital delivery, thanks to the lack of inventory-stocking hassles, but the actual content creators will ultimately want to handle digital delivery with end users directly.

So let's be realistic for a moment. Even if eBay's auction site is in a funk, burning more buyers and sellers than the summer sun, it would still be incremental to Amazon's margins. Hansell's suggestion is sound, especially when you consider the synergies. When Amazon doesn't have something in stock, it leans on its third-party sellers, but what happens when they don't have it either? Amazon and eBay would create a one-stop shop for all goods.

It won't happen, of course. Amazon has too much momentum to chance the extra weight, and eBay's pride won't let it relinquish the driver's seat to ride shotgun alongside a company with a smaller market cap.

Yahoo! and eBay are the ones that make better make-out partners. The two mojo-stripped bellwethers could use the mutual injection of excitement. That deal isn't happening either, though at least Yahoo! will be pressed into making a major move if things don't pick up there soon.

So don't look for eBay -- much less Amazon -- to be walking down the aisle anytime soon. They may be right for each other, but they'll never realize it.

Bid for more Foolishness:

eBay, Amazon.com, and Yahoo! are Motley Fool Stock Advisor newsletter selections. Read all of the original recommendation reports -- now -- with a free 30-day trial subscription.

Longtime Fool contributor Rick Munarriz is a satisfied eBay user with 173 positive feedback ratings to show for it. He does not own shares in any of the companies in this story. He is also a member of the Rule Breakers analytical team, seeking out the next great growth stock early in its defiance. The Fool has a disclosure policy.