Recessions are supposed to be boom times for yard sales and flea markets, so what’s eating eBay (EBAY), the world’s largest bargain bazaar?

Ebay shares hit a new five-year low Thursday as yet another analyst weighed in on the nagging deterioration of the auction giant’s business. Morgan Stanley analyst David Joseph downgraded eBay to neutral, pointing out that there are some troubling trends contributing to the company’s drooping e-commerce market share.

With fears that the credit crisis could throw the economy into a tailspin, Wall Street has been in a bit of a selling mood of late. When analysts point to signs of strain at eBay, investors don’t seem inclined to wait and see.

Foremost among eBay’s challenges, Joseph writes, is buyers’ shifting preference in favor of easy shopping, and away from the auction format. People like features like free delivery, shopping various selections within categories, and conveniences like one-click checkout. As the go-between agent in a transaction, eBay has limited control over these features.

As the analyst notes: Ebay is at a disadvantage in its “ability to compete in buyer experience.”

Given the changing tastes among shoppers, it’s not too surprising that eBay’s market share has dropped to 17% from 19% in the past two years. It’s also not shocking that Amazon’s slice of the business has grown to 5.3% from 3.7% in the same period, according to Morgan Stanley.

Other competitors have edged in as well. Local online swap shop specialist Craiglists has its fans. And while far from a runaway success, net giant Google’s (GOOG) Google Checkout purchasing system is an alternative to eBay’s PayPal service.

The weakening economy is having an impact on eBay also. Analysts point out that people are buying fewer items and at lower prices. So with less big ticket sales to add to the total tally, the overall average selling price is falling. And with the slowing sales volume in September, particularly in the U.S., eBay’s growth rate is “under pressure,” according to a Merrill Lynch report Tuesday.

To be sure, eBay has a solid position globally as the marketplace where sellers of cheap goods meet hunters of good bargains. And as a company, eBay is an upstanding financial citizen, with zero debt, $3.7 billion cash in hand, and a cash flow generation rate of $3 billion annually.

But as we saw this week with Monday’s 9% drop in the Nasdaq, even solid favorites in tech like Google, Apple (AAPL) and Research in Motion (RIMM) get trampled when people stampede for the exits.

Worries about eBay’s slowing growth and shifting consumer preferences certainly don’t encourage the highest bids in a market prone to panic.