The pair has just launched their website, which Nathan coded. Like eBay, it's designed to be a marketplace for just about anything with one crucial advantage: it has in-built social media functionality.

“It's a really easy way to get an online store up and running,” explains Nathan. “You can sign-in and add your first product in a matter of seconds,” he says.

The siblings had the idea for combining an online marketplace with social media after seeing people tag photos on Instagram of items they wanted to sell. “I thought there might be something in this,” says Naomi, a commerce student at the University of Sydney.

“I'm interested in marketing to sellers, which is where the idea for the site came from,” she adds, explaining that one of the reasons they came up with the idea for Sailr was because she had tried to sell items through Facebook. She found this difficult given there's no ability to accept payment through the popular social media platform.

“We wanted an easy way to sell to our friends and we thought there must be something in trying to sell through social media,” says Nathan.

So they started work on Sailr early this year, initially creating a landing page that included an explanation of what they were trying to create. They drove traffic to the landing page from Reddit, as well as with Google search engine marketing, using keywords related to selling online.

The landing page included a sign-up box, where initially 150 people entered their details. “We made contact with those people and from that got a feeling for what people wanted. [Going through this process] told us that we have a good idea,” says Nathan.

That gave him the impetus to start developing the underlying computer code for the site. “I learnt coding at school, then started doing more complex things,” says the talented high school student. He estimates he has spent 150 hours building the site, not including the hours the Feiglins have spent testing it, working mostly after school and uni and on weekends.

Now, Nathan and Naomi are focused on raising the profile of Sailr. They are concentrating on attracting sellers to the site. They have set up a Facebook and Twitter profile and intend to blog about how to sell on line and about interesting things that are for sale through the site.

Sailr has a very different business model to eBay. It's free to add four products to sell through the site, but people who wish to add more than four items pay a $12.99 monthly subscription, for which they can add unlimited products. At the back end, the site uses Stripe to process subscriptions, which is integrated with PayPal. This means there's no need for the Feiglins to handle credit card payments.

So far they have not come up with revenue projections, given they have little data on which to base them. “We're just hoping it will be successful,” says Nathan.

According to eBay, it has more than 145 million active users around the world, which indicates the immense scale of the market the Feiglins are targeting. It earned revenue of $US4.5 billion last year, a jump of 13 per cent compared to the previous year, which shows there's still healthy growth in online second-hand markets.

In Australia, eBay's closest competitor is probably Gumtree, which it also owns and which claims to be active in 11 countries. Other popular second hand sites include Trading Post, Craigslist, Cash Converters and Gray's Online. But nothing comes close to eBay.

So what chance do the Feiglins have of knocking eBay off its perch? According to Mick Liubinskas, head coach at business incubator Muru-D Accelerator, the Sailr team has a big future.

“There's a massive opportunity to disrupt incumbents by being focused and doing things they don't want to do. eBay is absolutely ripe for disruption; we've seen lots of tech businesses like [travel booking web site] Wotif go down recently. The world's a pretty open place at the moment,” he says.

The Feiglins claim they are not trying to compete with eBay. Says Nathan: “It's completely different. We focus on individual stores and sellers; it's a much more personal experience.”

The Feiglins' ultimate goal is to have people check Sailr in the same way as they would Facebook. But right now they are developing features for the site to make Sailr a better experience for buyers and sellers, for instance the capability for people to 'like' items listed on the site and save items for sale. They also want people to be able to favourite items for sale, as well as comment on them.

“The idea is for friends to be able to interact through the site,” says Naomi.

As for an exit strategy, the Feiglins says it's too early to be talking to investors, although in the future they would consider accepting venture capital if they do need additional finance.