Not all purchases are equal. For some of them you want to give a special touch and have a more personal communication with the user. After a user buys for the 3rd time in the app, have one of you sales team member reaching out to them.
When you use Liquid's Formulas, you'll select the audience, the action and the time.
Users that have already made 2 purchases
Send a message to the Sales team through a Slack channel so that they can act right away
Activity based on event “Buy”
Define a custom segment with users that have made 2 purchases (so the next one is the 3rd purchase).
Select Slack as the action and add the Slack channel link to where to send the message, i.e. the event that was done and the user that did it (automatically done by Liquid).
Make the action Activity-based and select the option that is triggered by the “buy-product” event.