Events Manager → Audiences → Custom Audience from Purchase events, 180-day window.
For low event volume, upload a customer email list alongside the purchase-event audience.
Ad set → Audience → Exclude: the purchaser audience. Placement: every prospecting campaign.
Repeat-purchase products may want a 30-day exclusion, not 180. Choose per product cycle.
Ads Manager → Breakdown by age/region is fine, but the real check is CPA falling on prospecting sets.