Consistent successful trading is the most popular way to garner trust from other members, but certainly not the only way. Think of how you'd trust someone on a daily basis offline...buying and selling goods can lead to a trustful relationship over time, but so can consistent "trustworthy" behavior (i.e. following through on your promises / keeping your word, adding value to the community, etc). Personally, I value a member's historical behavior more than I do their past trades.