It's quite clear on the trust page: "Positive - You think that this person is unlikely to scam anyone."

To me this means that even a successful deal shouldn't necessarily lead to positive trust, but on the other hand positive trust may be based not just on deals. If someone has been around for a long time, hasn't exhibited any questionable behavior, doesn't have a "get to the top at any cost" attitude - they might be considered "unlikely to scam". If someone has been applying for a bunch of micro loans or making tiny trades - they might not.

But this also means that it's going to be highly subjective. Everyone can have their own idea of how "unlikely" something is.