@slieneke the best way to do this is to create a flow that adds a custom profile property when someone places an order and then use that for your dynamic content condition. 

If you want to be able to back-populate the flow and update everyone who has ever placed an order, you’ll want to create a segment of people who have purchased and trigger the flow from that.

Otherwise you can trigger it from the placed order event and then just add a filter, has not been in this flow at any time so they don’t go through again and again for no reason. 

Then the only thing that flow does is:

Create Profile Property > Made Purchase > Yes

• I prefer to use text fields whenever possible because it’s easiest. 

In whatever emails you’re sending, the dynamic condition would be

not person|lookup: ‘Made a Purchase’ 

^^^ That’s essentially saying the field is not set

You can see common conditions here

You can also do inside a flow if you just want to know if they’ve placed an order since starting that specific flow like a welcome series. So you can do the conditional split right before the next email is about to send. 

Those who placed the order get the profile property and then you just reconnect the flow branch to the main one. 

Hope that helps!