Most Warhammer 40K: Rogue Trader Mobile Extractium items, at least a few hours into the campaign, come from a couple of colony projects. You should be able to see and complete these as early as Act 2.

The item trading in Warhammer 40K: Rogue Trader is primarily based on the reputation that you've earned for each faction. Other facets are also considered, such as your Profit Factor.