Looks really awesome! Have you heard of
Captain Blood? It looks like there are quite a few parallels with that game (I'd love it if you got the pictographic language communication system into your game!)
That seems quite simple - Some artifacts sell better to different Alien races.
I've discovered a race that I know is looking for a Hysenbob Discontribulator, so I can get a much better fee for it than you. Likewise, I've got a Dichloric Alloy Macrophotosabre (*) that I've not been able to offload for anything useful, but you personally know a guy in orbit around a certain star who'd love that sort of thing.
Maybe players trading directly with each other might not work, but if you get an AI working, it'd certainly make sense for two players to gang up against another player, and form some sort of trade agreement at the early stages of the game
The guys who made Captain Blood went on to form Cryo and make the original story based rts-ish Dune game.. One of my favourites and the one I replayed on my GP2x a few months ago (SegaCD version). Sounds very cool.
Again, the game isn't about the artifacts, it's about closing in on space. The players ability to move quickly through the galaxy and find aliens to talk to and trade with pretty much everywhere makes ganging up not make too much sense. Anyone who took the time to trade artifacts or information would quickly fall behind in the race.
I am thinking about a "co-op team mode", where two players play on the same team and share information and finances. But that information sharing has to be a co-ordinated effort throughout the game... information sharing isn't very useful as a one off event.
The comments here are exceptionally interesting and are getting me thinking about new game ideas... I'm just sure how easily they fit into the existing jigsaw.
The one thing becoming clearer to me is that with this many people interested in the game, getting networking multiplayer in there should happen a lot sooner rather than later. It's the last major component of my game that isn't polish that could radically affect how I'm coding the game (I've already tried very hard to ensure my "Ship" object isn't a static class (i.e. there could be more than one of them). I did some font and visual improvements yesterday, but I think I'll start working on that next. Actually playing multiplayer is going to open up a lot more game ideas...