If there was ever a time to dethrone Craigslist, it’s now. The company’s bare bones online marketplace has been a source of “macro-angst” for years. Yet no one’s been able to displace it because that’s where the buyers, renters and job hunters are. Without those people, a marketplace is just a sad warehouse, which is what’s become of many would-be Craigslist competitors.

The need is even greater now that Craigslist has gone after developers like Padmapper and 3Tap who’ve built their own improved user experiences around its listings. (If any one of the new players does manage to create liquidity, the best chance they have at unseating Craigslist is through mobile.)

And yet, as our intrepid intern learned earlier this summer, it’s not easy to unseat the marketplace incumbent. On the myriad of new marketplace sites, nobody wanted the PandoDaily staff’s perfectly acceptable secondhand stuff.

Zaarly has succeeded at creating all-important market liquidity by starting with the demand side. Users request something, and anyone can agree to deliver on the request.

Grabio launched around the same time as Zaarly but was not as well-capitalized and slower to grow. The app initially opened up nationally but then realized that without penetration, the market was entirely one-sided in many regions. So Grabio scaled back and decided to focus on creating liquidity. Grabio decided to start by targeting college towns, too, since they have a high turnover of students moving in and out of the region each year.

Grabio built a feature called “Wish List,” which is perfect for people that are making considered, typically local purchases like furniture.

Now, if you’re in the market for, say, a couch, you can add a few specifications to your wishlist and do nothing. When you’re in the same region as a new listing that fits your interest parameters, you’ll get a push notification. For something like, say tickets to a concert, this is clutch. And for goods, it helps make your shopping journey a little more passive.

The key to this working is that it doesn’t drain your phone’s battery. Rather than send constant requests to the phone’s GPS in the way that now-maligned location-aware networking services like Highlight did, Grabio takes a different approach. The app simply monitors when your signal changes to a different cell phone tower. If you move to a new tower, the app wakes up and does a search. The app doesn’t know your exact location, but it can be accurate within a one mile radius.

Founder and CEO Horatiu Boeriu says Wish List has quickly become the most-used feature on the app. He’s okay with expanding slowly into new regions because it’s important to keep engagement high. “We could buy users, but if they don’t use the app regularly then its pointless,” he says. Boeriu pointed to Viddy, which skyrocketed to the top of Facebook’s app lists because of a viral news feed integration, but has now fallen from popularity as fast as it took off.

“I’d prefer to have 100,000 users with 80% engagement then to have one million users with 20% engagement,” he says. Right now he’s got fewer than 100,000 even, but usage is growing slowly and steadily. By starting with demand, rather than a warehouse of goods with no buyers, he’ll have a strong chance of taking a piece of the crowded market.

The next potential step for Grabio is interesting from a business model perspective. The company would like to start integrating first-hand goods (read: non-garage sale items) from retailers. Meaning, if you’re in the market for a certain type of television, Grabio will tell you when you’re near a Best Buy which sells the television for the lowest price in a 100-mile radius. If done well, this could be a great way to generate hot leads for retailers. Or it could be annoying as hell (eg, I already know Best Buy sells TVs, I know when I’m near a Best Buy location, and I’m sick of hearing about deals and discounts!). I can’t decide. Either way, Grabio has the right idea by creating Google’s holy grail of “intent” in a sector that’s heavy on sellers and light on liquidity.