Relatively speaking rpg maker games are a niche market its not super sizable compared to other markets, to put it simply people love visually interesting, unique, and fun gameplay experiences.
As for the first one rpg maker games tend to look really similar to people if store packs (and especially the rtp) is used and some people won't touch them because of that; people want to feel like its something they couldn't have made themselves (a lot of rpg maker game buyers are also users). Adding in artwork that separates your game from others is an excellent way to stand out in a sea of rpg maker games; same thing applies to music and sound-effects as well.
As for the other two this depends on how you develop the game; sure a lot of people can make a good standard rpg but I think if you really want to stand out you need to show something in it that isn't the norm; maybe it has a unique gameplay style or your story is top-notch, etc...
Rpg maker games do sell for some people but it should be said right away that it isn't an easy path to success you got to build your name up, get a fanbase and improve your quality as you get experience and feedback as you go. You aren't going to be able to buy that fancy car with your first game. Indinera founder of Aldorlea Games makes his living from rpg maker games so it can be done its just not the exact case for every person and it depends on the amount of time and work put into it.
It should be noted what makes an rpg maker game sell can vary considerably some people love a classic game with tons of gameplay length, while other times something short but visually interesting can sell like this game:
http://freebirdgames.com/to_the_moon/.
I've seen commercial rpg maker games with mostly rtp stuff seem to get good reviews because everything else was on point and I've seen and played rpg maker games that have sold well but weren't even all that fun to play to be honest or had a lot of issues still mechanically, they got sales though because they stood out and got people interested to try the game. As artificer said part of being a developer is also figuring out how to sell your game, how to get your target market interested in it.
Lastly personal advice if you are working on your first project or two don't try to go commercial with it. It can take considerable time to learn the skills needed to work with the database and to get used to what it is like to actually develop a game; some people have the maker for years and have never finished a game due to not being able to stick with a project. I've had and been using Ace for over a couple years now and only just start to feel like I could be seriously ready to try my hand at a commercial release for a game; I've only finished two games to be honest in that two year span but the rest of it was spent learning and improving with the maker, seeing how others did things in their own projects. Had I seen the first game I finished now released as a commercial game I would think it a joke.
Anyway best of luck. For more in depth conversations about commercial games there is a section on the forums for commercial topics.