Hi,
Although donating does not equate to trustworthiness, its a great way to start providing some sort of opportunity cost to smaller trades. I.e. someone will more likely trust you with $20 if you have donated $50, as the 'cost' of you scamming is $30.
From then on, vouches and trust kinda snowball.
You may also choose to gain vouches by providing free services, such as skilling or questing. People will vouch in turn for your honest work.
Goodluck!