When setting a success metric for buy and sell groups, it's important to first define what success means for the particular group. Is it increased engagement, more transactions, or higher revenue? Once the goal is established, the success metric can be determined. For example, if the goal is increased engagement, the success metric could be the number of active users per week. If the goal is more transactions, the success metric could be the number of completed transactions per week. And if the goal is higher revenue, the success metric could be the total revenue generated per week. It's important to track the success metric over time to ensure that the buy and sell group is meeting its goals and making progress towards success.