When all the customers are going to the supermarket to buy their fresh veg, you can set up your own puny little stall in the alley, but guess what, no one's buying because they can't even find you.
Apple does not allow you to sell iOS apps outside its app store, and there's every reason to believe they are pushing the desktop market in the same direction. Why wouldn't they?
Under the big Available on the Mac App Store button it says:
"Try before you buy!. 15 day trial version also available."
(You did see that this thread is about a Mac app right? Not about an iOS app. I do agree that the situation there is very different.)
This is incorrect in a minor, but fundamentally important way, I think:
Apple does nothing to prevent you from selling outside their store. What they've done is make it difficult for the average iPhone owner to buy from you.
You need to target a specific demographic that can overcome those obstacles and will pay enough to make it worth your time.
In practice, that probably means enterprise.