Good space games

X3TC and X3AP are great games, you can do alot in them but there's a huge learning curve.
And there are alot of mods out there for them and a few great overhauls that add alot of great features to the game.

my favorite being Litcubes Universe.

Mass Effect 1-3

If you want a bit less trading & fleet management, but still want to have options in what to do, then check out Wing Commander Privateer.

Freelancer was one of the best games of my childhood, give it a try!

edit: I know it's not on steam, but give it a try anyway :)

Space Rangers HD: A War Apart

X3:TC and X3:AP are definitely good ones to get into. X3 Rebalance Mod is the mod that I've been using for Terran Conflict.

As for some other games that you could check out;
Evochron Mercenary
Darkstar One - (I haven't played it, but I believe you can do trading and all that stuff)
Freelancer

I can't think of much else at the moment. But, hopefully you can find a game that you'll love!

Privateer

On sale right now on GOG

http://www.gog.com/game/wing_commander_privateer
http://www.gog.com/game/privateer_2_the_darkening

http://store.steampowered.com/app/107200/
I like this one

Elite: Dangerous is fun, but don't expect a structured story. A lot of new players buy the game and then are completely overwhelmed because the game doesn't tell you what to do. You start with a ship and some credits and the rest is up to you.

Ever checked Freespace?

Endless Space is pretty fun. But you control a whole empire instead of a fleet. Some rpg elements in it too.

FTL is a great rogue-like space sim where you control a crew and it does have some rpg elements as well.

I like Endless Space. It's not exactly what you are looking for, but it's worth a look.
It's a 4X Strategy game set in space. I normally don't like 4X games, but this one dragged me in for dozens of hours.

Starpoint Gemini and Starpoint Gemini 2

http://store.steampowered.com/app/34810/ awesome*

*I rated that game "awesome" when i was younger, atm could be just ok or meh.

Starflight 1 and 2 !

I just have to mention the obligatory Star Citizen. If you can hold off for now, you might just binge in a couple years. You can try out any ships you buy now and do a little dogfighting.

My favorite in the space genre is Wing Commander 4, but it's a little dated at this point sadly.

EVE Online!

Try 21 day trial.

Also kerbal space program is lots of fun, tough less on rpg/adventure side more on NASA in broken mirror simulation. And you learn how stuff in space really works ( i think, for some reason being good at it isnt enough to get job at NASA)

FTL is good and pretty cheap