IMO Default Trust is half broken, so i would disagree DT is most important metric. Any feedback from DT should be used as reference rather than used to judge someone directly without any further research.
So my question is - how can members improve their trust ratings if there are no business trading opportunities in the forum?
Fighting scammers, not being a scammer, time, and getting to know other members on a level that's more than superficial. Those things help over time--but there
are trading opportunities on the forum, e.g., in the Currency Exchange section, although that's not a great way to obtain green trust from a DT member if that DT member is prudent as far as handing out positive feedbacks goes.
Many DT who perform trade nowadays left neutral feedback with positive comment since some people attempt to abuse it.