LOS ANGELES — Recently, the chairman of online auction site www.Bidz.com forecast good news ahead. The company was headed for another blowout quarter, David Zinberg said, with sales topping the already-rosy projections made in November.

The announcement was designed partly to quell questions that had been swirling around the company for months.

The questions involve, among other things, what critics say are irregularities in bidding patterns on the site, as well as persistent customer complaints that have led the Better Business Bureau to give the company a grade of "F."

Critics also have raised questions about www.Bidz.com's relationship with Saied Aframian, a primary jewelry supplier who served time in prison for receiving stolen property.

Zinberg, an immigrant from the former Soviet state of Moldova, attributes much of the criticism to short sellers — investors betting on a drop in a company's stock price, and who aren't shy about trying to push it in that direction.

Indeed, public records indicate that investors had sold short nearly 3.9 million Bidz shares as of Jan. 31, a significant proportion of the roughly 10 million shares available for public trading.

Still, the questions appear to be having an effect; www.Bidz.com's shares have tumbled more than 30 percent since Zinberg's upbeat Jan. 14 announcement.

As Zinberg tells it, there's no mystery to the company's success. He says Bidz is enjoying strong growth because of the allure of its auction style and management's hard work.

"We come here at 7 a.m., we're the first ones to show up, and we love what we do," he said at the company's 52,000-square foot headquarters and warehouse in Culver City, about six miles from downtown L.A.

But there may be reasons for the stock's downdraft other than short sellers' innuendo. Nearly half of Bidz's shares are owned by Zinberg and his sister, Marina Zinberg — and they have been selling relentlessly, unloading 210,000 shares for nearly $2 million since Aug. 15, regulatory filings show.

Also, in recent months, the company's two largest institutional stockholders dumped their combined 1.5 million shares. Neither would discuss their reasons for divesting.

Zinberg says his stock sales make up for his decision to take no salary from Bidz. Nevertheless, he cut the pace of sales back sharply starting in December.

The successor to a chain of 12 Southern California pawn shops Zinberg owned in the 1990s, Bidz offers fast-moving online auctions, mostly for cheap costume jewelry but occasionally featuring precious gems and watches it says are worth tens of thousands of dollars.

The auctions start at $1 and carry no reserve price, meaning that bids don't have to reach a minimum level to win an item. Unlike eBay, which brings individual sellers and buyers together, Bidz is the seller of almost every item on its Web site.

The company's leading supplier of jewelry is Los Angeles Jewelry Production Inc., a downtown company managed by Aframian, who also is listed in regulatory filings as a co-owner of the company.

An Iranian immigrant, Aframian served a two-year term in state prison in the 1980s for receiving stolen property. In a 2006 civil suit, a New York jewelry wholesaler accused him of stealing $285,000 in jewelry that he had left overnight in Los Angeles Jewelry's office safe. The case was settled on confidential terms.

Aframian, who would not comment, is the third-largest shareholder of Bidz, behind the Zinbergs.

As for its customer service, the Better Business Bureau says Bidz's "F" is based on customer complaints of shoddy merchandise and misrepresentation of items' quality or value.

A frequent target is the company's "compare" price, a dollar figure Bidz posts for many auctioned items that purports to represent the maximum price that has been advertised online for items that are "similar or the same."

Karen Leonard, a Philadelphia-area resident who filed a complaint last month with the Better Business Bureau, said she paid $800 for a diamond ring on which the "compare" price was $10,769.

After receiving the shipment, "I knew immediately that it couldn't be worth $10,000," she said. "But the description (on the Bidz Web site) made you think you were getting a really good deal."

She said Bidz has refused to refund the full purchase price on grounds that the item was "as described" and therefore subject to a 15 percent restocking fee.

Bidz.com often offers buyers an "appraisal" of their item for a $33 charge. The reports are invariably produced by L.A.-based American International Gemologists, which acknowledged to the Los Angeles Times that it does not physically inspect every item for which it provides an appraisal.

That raised a red flag for Jeanine Woodling, director of education at the nonprofit International Society of Appraisers. For a solid appraisal, "It's important for them to actually be able to see the piece," Woodling said.

Critics of Bidz, including some who claim to be customers, say they suspect shill bidding has driven up the prices of many auctions. This allegation was published online in November by Citron Research, a Los Angeles Web site run by short seller Andrew Left, and has also appeared in postings on www.Ripoffreport.com, a Web site that collects consumer complaints.

As evidence, they cite auctions in which prices were bid up to implausible levels on items available at retail for much less. Late last year, for example, buyers mysteriously bid as much as $11,000 for big-screen TVs that retailed elsewhere for as little as $800.