It's been a defining year for SeatGeek: The ticket-search startup raised a $62 million round of funding, brought its New York City headcount to 100, and is in the enviable position to be designing itself a new office in Manhattan's SoHo neighborhood. But over most of the past year, staff scattered across three currently crowded offices have been heads-down on building the company's new capability.

SeatGeek is known as a platform for buying sports, concert, and theater tickets (it lists and compares prices from hundreds of other ticket companies, and is often thought of as "Kayak for ticket sales"). As of today, it's also a prime place to sell them: It's opening up what it calls SeatGeek Marketplace, and the ability of any user to sell their event tickets they've already purchased.

Can't make it to that Billy Joel show at MSG you bought tickets to months ago so you could take your mom (oh, darn)? The site and iOS app help you list them by uploading the .pdf the original ticket. It will suggest pricing based on its existing algorithms, which are used, among other things, to create its existing "deal score," a ranking of how good a value a given listed ticket is (as opposed to other sites, which rank by price alone). And those suggestions will change based on demand. Or, users can set their own price, and pay a fee to list the ticket. Or, for no cost, a user can send the tickets to someone they already know--request payment for them, or give it for free.

"One of the reasons people don't buy tickets is they are afraid that when the date comes they won't be able to go," one of the company's founders, Russ D'Souza, explains. "This takes away some of that worry--and makes it really easy to transfer tickets, as it has a social element to it."

SeatGeek was founded in Boston in 2008, a great year for Boston sports teams; a horrific year for finding desirable seats at affordable prices. It has grown into a ticket aggregator and competitor to StubHub, Ticketmaster, and Zvents.

If this works, SeatGeek will be seen as an admirable master of the not-quite pivot. Call it a "defining launch:" The move that doesn't seem hugely significant at the time, but comes to define the business. It happened once before.

When its founders decided to put resources into making a mobile app that would perform all the functions of the website, and hopefully allow ticketholders to electronically scan their codes at stadiums and other venues, they thought it might be a nice bonus for users. ("It's silly to print a .pdf when a barcode is just a simple representation of an integer," says D'Souza.) After all, a purchase of sporting event or broadway tickets is really large--the average SeatGeek purchase is $250, which as of a few years ago seemed like a lot to be shilling out while just on your phone.

Within months of the late-2012 iOS app launch, it was clear that their best customers loved it more than the site. And today, the company is known for its "mobile first" strategy, which is supported by the fact that 70 percent of sales take place on mobile now.

This new move is a big customer-acquisition play. An existing user can transfer tickets to anyone, via an email address. To accept, one must log in. And a social-network of sorts is forming. So is a vibrant recommendation engine, that can ping users via text or email when new tickets to events they are interested in show up on the platform.

John Locke, a partner at Accel Partners and advisor to SeatGeek, says he thinks the mobile ticket-sales and transfer capabilities are an innovation that's long overdue.

"I think we all have lived through experiences where we're trying to sell a ticket on craigslist--it's always been such a nightmare of a process. You buy a ticket outside a concert, you hope it's legit--no company has made the process of transferring a ticket a couple of clicks," he says. "It's crazy that this hasn't been able to occur until 2015."

He said he urged Accel, which has invested in Kayak and HotelTonight, to back SeatGeek, due to the fact the company resembled these mobile-ready marketplaces. 

D'Sousa founded SeatGeek along with his former Dartmouth College roommate, Jack Groetzinger--with whom he'd tried out a couple of other startup ideas that didn't gain traction. But SeatGeek took off early--by 2009 the company moved to New York. Since, it has since been growing and stretching out through three different offices, where employees are packed tightly today, but enjoy perks like free CitiBike memberships and a monthly concert-and-event stipend--to spend on SeatGeek, of course.

"I do a lot of 'research' at shows and sports events," says D'Souza.