Hello and welcome to the combine. I hope you will get a great time here. First the combine is not a finished game, its still under development. to continue with your questions.
1) Items are not fully implemented at this point. If you check the simnews you will see a post a few days ago showing that progress is being made in the area of items and that they should be one of the coming main releases.
2) Ships, vehicles, spacestations, facilities,stocks, rawmaterials and NPCs is what you can buy and sell at the moment. This is mainly done in 3 different areas. The first would be on this site under the RPG board. There you will find a thread called commerce. The second way is to get on the combine IRC server and join the traders room there, #swc-traders. the last way would be to check out www.swc-gm.com which is a site many uses to buy and sell things
3. At the moment quests are not added to the game. Its being worked on by the rules and development teams. You can do roleplaying if you check out the link to the white scenario board. Most people are with a faction which helps you to get started and meet people. The game is not designed for people to be freelancers. The few freelancers there are have been in the game a long time.
4) same as with quests, monsters or animals/creatures are being worked on. And once implemented a quest could very well be to go and kill creature X and Y.