Looking for something like flippa.com, but for mac apps. If there is none, what do you think would be the best way to sell an app ? I'm thinking about letting another developer take over Hands Off!, as I'm focusing on a new app.
You could checkout http://appbodega.com. It's an App Store alternative and works as a standalone app that you need to install on your Mac. You can sell your apps via Bodega as a developer. Though you cannot "Buy and Sell" apps. I think most licenses for apps would normally limit resale of apps to another buyer. So something on the exact lines of flippa.com may not be possible due to license restrictions.
So, Apptopia (https://www.apptopia.com/) doesn't list Mac apps today, but if you do have a buyer lined up, they'll do the transfer and provide escrow on the cheap.
