Hi Try this website http://www.sellmyapp.com/sell-android-apps/ it looks as if it's exactly what you are looking for
If the app is specific to a certain market segment, then find corresponding forums that cater to that market segment and buy advertising in the forums pages. Or use other means like signature links in your forum posts. Putting it up in a "general store" website like sellmyapp and waiting to be "discovered" can make for a long, long wait.