In the upcoming redesign of Stencyl.com (timing of this is still up in the air), I'll make sure we get a proper page going for extensions as part of a brand new "Developer Center" section of the site.
It would be a stretch to call it a "market" since no money's going to change hands (though you will be able to accept donations/tips via Paypal), but the presentation may be market-like. I hope that this further encourages the building of extensions for our platform.