Prosper Marketplace Inc., which is developing a secondary market for loans originated through its Web site, also plans to let banks use its system to resell their loans.

Observers said that such a service, which the San Francisco company disclosed in a filing with the Securities and Exchange Commission Friday, would let banks reduce their capital requirements.

Prosper initially registered with the SEC last year to create a secondary market that would enable its users to trade the notes for loans made through Prosper's peer-to-peer lending service.

Friday's update to that filing included details about expanding the system to permit financial companies to sell their loans to Prosper's users.

The system would "enable our financial institution members to offer previously funded loans for sale by posting listings on the platform indicating the remaining principal balance of the loan and the interest rate the borrower is obligated to pay on the loan," Prosper said in the filing.

Prosper is in a quiet period because of the registration and would not make executives available for interviews.

Bobbie Britting, a research director in the consumer lending practice at TowerGroup Inc., a Needham, Mass., independent research firm owned by MasterCard Inc., said that banks would find Prosper's secondary market an appealing way to get rid of some loans, especially in the current market.

In recent months some lenders were "selling their current loans at a discount, thinking it would be better to get them off their books" at a lower value than to keep them, Ms. Britting said, and Prosper's site could help facilitate such transactions.

Ms. Britting noted that such sales through Prosper "are nonrecourse to the financial institutions, which is very good these days, because it means they don't have to maintain the capital requirements for them."

Significant differences exist between the loans originated through Prosper and those it would offer from financial institutions. For one, Prosper users that buy bank loans would have less information.

"We do not obtain any documentation on the borrower's ability to afford the loan," the filing said, though Prosper would provide identifying information such as the borrower's name, address, and Social Security number.

Prosper stopped facilitating new loans in October as part of its effort to create the secondary market, but people who borrowed through the site in the past were required to supply personal details in their requests, and Prosper provided lenders the Experian Scorex Plus scores of potential borrowers.

Another difference is that all loans created through Prosper are unsecured, but banks would be able to sell secured loans through the secondary market, the filing said, though such loans would become unsecured in the process; buyers of the loans would receive no interest in collateral associated with the loans.

Edward Woods, a senior analyst for the Boston market research firm Celent, the financial research arm of Marsh & McLennan Cos. Inc., said Prosper's evolving plans for a secondary market seem to break from the company's image as a lending service for individuals.

"P-to-P was about peers and the social aspects of living and finances," Mr. Wood wrote in an e-mail Friday. "How this aspect of their offering is described, it is at a minimum" business-to-consumer and could even be a business-to-business transaction if a bank is on the buying end as well, a notion that Prosper has floated in the past.

"When I thought about banks playing in the P-to-P secondary market before, I assumed that they'd be trying to buy a portfolio of loans; I didn't think that they'd be selling them," he wrote.

He also said that Prosper has come under fire from regulators, which have deemed its loans unregistered securities; requiring the company to register all the loans made through its site would "raise the stakes in regard to suitability standards for lenders," Mr. Woods wrote, and could shrink the pool of potential users of its secondary market.

Opening the service to banks would make more people and companies eligible to sell loans through Prosper's secondary market, he said.

Lending Club Corp., a Sunnyvale, Calif., peer-to-peer lender that introduced a secondary market in October, for example, had to restrict who would be eligible to resell loans.

Lenders in most states must have gross income of at least $70,000 and net worth of at least $70,000 (excluding some possessions, such as home and automobile), or they must have net worth of $250,000 with the same exclusions but no income floor. (In California, these requirements are even stricter; lenders must have six-figure incomes and net worth.)

Renaud Laplanche, Lending Club's chief executive, said that his system cannot be used to sell loans originated elsewhere but that this does not exclude banks from participating.

"Both in the secondary market and on the main platform … we have a variety of both individuals and institutions," he said.

However, the processes are not separate for banks and for consumers; all participants use the same tools to make loans through Lending Club's platform. "When you look at what's happening on eBay, you see pretty much the same evolution," Mr. Laplanche said. As eBay Inc.'s online auction site grew, more businesses began to use it as another channel to reach consumers.

Mr. Laplanche would not comment on Prosper's plans but said his company lacks the ability to bring in existing loans from banks. "It's really not set up that way, at this stage," he said.

The reason is that, for loans to be traded on Lending Club's secondary platform, they must first be registered as securities. All Lending Club loans created since October are registered and thus eligible to be traded, but there is no mechanism for registering loans that were originated elsewhere, he said.