Besides Steam and GOG, the Humble Store may be a good place to sell it, since a purchase there can give people both the DRM-free version and a Steam key (when Greenlit).

There's also GamersGate and Fireflower Games (the latter especially seems to be receptive to small indie games such as this).

 

Plus, there's always itch.io.

Another vote for Humble and itch.io.  Both seem to have pretty low bars for entry so it shouldn't be all that hard to get the game on them.  Humble also has the added benefit of having an embeddable widget you can include on the game's web page.

 

Green Man Gaming also sells games but I'm not sure if they're just a vendor for Steam keys.  Might be worth investigating anyways.