Hey Jack, I’m sorry for the slow response.
Breww will assume that it is the source of valid information for the audience and may not play very nicely with other platforms feeding into the same audience, but there is something you can do to make it play more nicely.
There’s a setting named “Allow Breww to remove list members”, see this screenshot:
With this setting NOT ticked
In this case, Breww will look at everyone that it thinks should be on the list, and update their “member information” on Mailchimp to match what Breww thinks is correct. This may involve adding new members (if they were not found on Mailchimp), and also updating existing members’ information - such as name, tags, last order date, etc.
With this setting ticked
Breww will do exactly as when this is not ticked, but it will also remove members completely if they are not on Breww’s version of the list. This means that if Sellar were to add a customer to the list and Breww didn’t know about them (based on email address), then Breww would remove them from your list for you. Clearly, this would be unhelpful in your case, so you most likely wouldn’t want this box ticked.
Having this box ticked allows for some clever things in Breww/Mailchimp, but they’re not usually relevant if you’re trying to use a single audience in Mailchimp (and have other platforms like Sellar also syncing to it). For example, with this ticked, you could make a dynamic customer list in Breww for all customers who have not ordered for over 60 days but have ordered in the last year, then get this to sync to an audience in Mailchimp. Breww would then keep this list up to date at all times for you automatically by adding & removing people from the list as and when they fall into or out of the criteria. If someone placed an order, Breww would remove them from the list/Mailchimp audience shortly after. You could then have automated email sending schedules in Mailchimp for members of the list.
I’m sure we could explore updating the Sellar integration to bring over marketing preferences to Breww, for us to then sync to Mailchimp, but as Breww doesn’t have its own concept of opted-in/out of marketing yet, this would be a big task. If you’d like this, please do post it in the feature requests section and then we’ll be able to track it through to implementation and allow others to vote it up the priority list.