Solid, well defined principles that are followed as much as possible. Not more variables to an equation.
Who's going to enforce those principles and how?
The same way it is enforced now, only with less steps. The community, only we have a clear set of rules we all agree to follow as opposed to a murky equation with ambiguous terms and tons of exceptions.
If you want to fix the trust system we need 2 tiers of trust. One for trade EXCLUSIVELY where a loss is incurred and can be documented in some way. The other for peoples opinions and feelings about everything else. Trade trust would be calculated into the average, and the other trust would not, simply amounting to a public notice.
The primary issue with the trust system is it is used as a political weapon to not only punish people for speaking out about bad behavior, it allows those same people punishing people for speaking out to destroy anyone lower than them for doing so. This is difficult to do if the trust is restricted to trade. People can retaliate this way all day with meaningless gripe trust and not affect their overall rating calculation. Anyone they trade with who is overzealous with a rating automatically has the same opportunity to leave their own negative rating that is calculated.
You could even potentially automate this to an extent by having an internal forum version of signing an agreement, enabling both to leave a rating at their will. IE I agree to enter into a trade with Bob, Bob and I both click a button confirming we are engaging in a transaction, opening the possibility of a weighted rating to be left. In this way the only weighted ratings left are from those with direct engagement, and not as a political tool by 3rd parties.
This doesn't need to be rocket science. A couple check boxes and a small description of its function to enable a one time token to leave a rating. I really don't even think amounts need to come into calculation as the individuals can include that information in the comment area if they so choose. At the end of the day the trust system should function as a guide for users, especially newer ones. Either way it should not be used as a substitute for due diligence. However if the trust system itself causes more conflict than the fraud it was designed to circumvent, then what is the point? Simpler is better in this case IMO.