welcome to the forums barca
1st question:
if your talking about darkbasic pro then yes you have to code the games and you can add code to it later to improve it. but if your talking about 3d game maker of fps creator then you cant add code unless you get the source code for it.
2nd question:
once you have bought these products you are allowed to sell anything made with that product. except for dark gdk which you have to buy a separate license to sell your creations.
3rd question:
yes its perfectly possible but don't expect to be able to make them straight away. your going to need lots of practice and have knowledge of the software/language.
hope this helps you.