There's no reason you can't sell such a game on steam through greenlight.

You'll need to spend a lot of time on high production values.

This means high quality artwork, sounds music, a good plot etc, plus a good trailer, marketing campaign, adverts etc.

Without these any game released on steam is dead in the water no matter what engine or toolkit it's made in.

You'll also need to promote heavily on twitter, Facebook etc and you need to be prepared to put money in to advertising to get sales back out the other end.

Good luck!

Oh... I have to do all that? I can't just slap something together with the elements that come with the program and plop it on steam and cross my fingers?

No one's stopping you, but you better be really good at crossing your fingers.

If you do all the things I recommend above you have a good chance of breaking even and at least making more than you spent on adverts and promotion. This is classed as a successful game and you could call yourself a successful game developer.

If on the other hand you throw a game nobody has ever heard of onto steam and cross your fingers you'll have better luck buying a lottery ticket than making anything as it won't ever even get enough votes to get greenlit...

essentially, I have no money.

Just thought I'd point out too, that putting a game onto greenlight costs $99 or so, and this itself is still no guarantee of it being released as you must then get a threshold number of upvotes to get greenlit...

Oh... I have to do all that? I can't just slap something together with the elements that come with the program and plop it on steam and cross my fingers

No you can sell it, it's just a question of whether it will be successful or not. If you use the built in graphics, sound, and music it's less likely that you'll have sales... you need to differentiate yourself from the pack. Things like custom battle systems are of course a plus but making post on rpgmaker related forums, rpg forums, having a trailer or two will go a long way in getting the word out. It comes down to "why should I play your game?"

edit - oh and yeah there are quite a few rpg maker games on steam, To the Moon for example. So unless the license has changed for this newest one I think you're alright legally.

RPG Maker games that use RTPs are generally not received well, since they all look very much alike.

There are a few RPG Maker games that distinguished themselves and did a bit better, such as the Cherry Tree High games and Stray Cat Crossing.