You can sell what you make royalty free; meaning you don't have to pay TGC a percentage of what you make.
Of course there are conditions.
1) You may not use other users or developers resources (ie. Music, Models, textures or any other created content) with out that person or developers consent. Freeware, open source and freely given items are considered permissable.
2) It helps if the game is original but remakes are okay, but be careful of using trademark characters or story elements in your game or creations. Commercial developers may send you a cease and disist order if you try to make money from thier characters or work. This is considered a requirment to protect trademarks and copyrights.
I'm sure there are more but these are the main issues that you have to keep in mind.
In Memory of My Dad.
I miss you very much.