Why is this an issue? They can just set the price accordingly so they can get the same profit after deducting the commission fee.
And since they can release the same addon in other sites, they can set their own price in there as well.
So if they publish the addon for $20 in Marketplace, then can also publish the same addon in their direct website for $10 and they still get the same $10 income regardless of where the people buy it from.