Recettear was pretty charming if simplistic.

This. The space sandbox genre has really been a pale shadow of itself and hasn't developed in any significant way since Frontier came out. First Encounters refined it slightly, but it was a buggy mess. The X series is great, but it has no planetary landings, which makes it feel a little unspectacular somehow.

I'm still waiting for a space sandbox trading/exploration game that provides full physics simulation à la Frontier, great economics à la the X3 series, full planetary landings and the ability to walk around and out of the ship seamlessly into cities. I want to be able to walk into my cargo hold and see my cargo while my ship is flying on autopilot. I want to be able to walk to my quarters on my own ship and see the planet my ship is orbiting.

Rodina looks to be closest to this at the moment, although it seems to be lacking the civilisation aspect of it. I do think we're at a technological stage where this is possible though.

Colonization! Probably the best non-RPG game on GOG, and trading is a huge part of it.

Space Rangers 2, probably one of my favorite games ever now. Trading is a big part of the early game. Sadly it's not for sale at the moment.

Port Royale 3
Patrician IV
Sid Meier's Pirates
X3
The Guild
Capitalism

Can recommend all of these for a trading experience of some sort at least.

Recettear as mentioned above is sort of interesting to be the merchant for a change.

Evochron Mercenary may fit the role but I haven't been able to put time into it yet.

He did say it didn't have to be the main focus :)

I'm surprised no one has mentioned Imperialism. I love this game. You have to trade to succeed and it is a lot of fun. You have to think about supply chains, raw materials, processed materials, labor, lots of stuff. I highly recommend it. Imperialism2 is worth playing as well, but Imp1 has more trading in my opinion.