We already know eBay is a goldmine for picking up bargains and casting off your old unwanted junk. But in fact there are billions to be made buying stuff on auction websites, a new study has found.

In 2003, eBay buyers saved themselves $7 billion (£3.5 billion) using the auction website over traditional retail stores, a study from the University of Maryland's Robert H. Smith School of Business found. This amounts to around 30 per cent of the total $24 billion spent on eBay that year.

eBay users save billions

This figure has increased over the years - in 2004, savings of $8.4 billion (£4.2 billion) were made. And the forecast for 2007 is that eBayers are expected to have saved around $19 billion (£9.6 billion), Reuters reports.

The amounts have been calculated as the difference between the top price buyers were ready to pay and what they actually ended up paying, or what economists call 'consumer surplus'.

"Consumer surplus is usually very hard to measure," said study co-author Galit Shmueli. "The problem is that it is hard to ascertain how much a winner or a bidder or a user would have been willing to pay for a certain item."

Not all eBay secrets revealed

The highest bidder only pays the next amount up from the second-highest bidder, so no one except the highest bidder and eBay knows what the buyer would have been prepared to pay. eBay is not giving out such data.

Good news for eBay buyers then, but what about sellers? Are they setting their prices too low, leaving billions of dollars in potential revenue lost? It's something to bear in mind next time you try to flog your old cast-offs on the auction website.

eBay is also said to be introducing changes designed to make its auctions more appealing to sellers while also making it easier for consumers to find items they wish to buy.