MoPub, an Accel Partners-backed startup from former AdMob and Google employees, is launching a marketplace where developers can auction off mobile advertising inventory in real-time.

Publishers can sell their advertising inventory while being able to control the type of ads that appear in their apps (if they want to block ads from specific brands or competitors). MoPub compares it to a “virtual trading floor” for ads where buyers come into a real-time market and look for demographics that match their needs.

“Any publisher from long-tail developer, who just wants to drive a lot of revenue, all the way to Tier 1 publisher like Conde Nast can clear their inventory through our platform,” said chief executive Jim Payne. “We provide a level of transparency that ad networks are not capable of. Ad networks have an opaque process. It’s a black box: the publisher gives their impression to AdMob and AdMob just sends them a check.”

Google’s DoubleClick has a comparable product for online display advertising called Ad Exchange or AdEx for for short, and Velti-owned Mobclix launched its own real-time bidding platform for mobile ads last fall.

MoPub says it has 650 publishers on its platform at the moment, including Fluik, the Canadian developer behind the game Office Jerk. Publishers can decide whether to show the name of their apps ahead of time to bidders and advertisers and they can block ads from particular brands or advertisers. They can also set price floors and caps so that ads aren’t shown too frequently to the same user.

The San Francisco-based company has raised over $6.5 million in venture capital from Accel Partners and Harrison Metal Capital. It also has a few other products it offers to developers. The company has a mediation layer, which means that it can help developers juggle multiple ad networks by swapping in the highest-paying ads in real-time. There’s also a platform for app publishers to directly sell their ads. (When developers sell their own ads, they usually get to keep a higher revenue share than if they handed over their inventory to a network that sells on behalf of them.)