Hannah:
Here's an approach that might work. Bear in mind that I haven't actually tried this, I just know there's an AppCloud app that should help.
The free Data Card Evaluator app lets you evaluate a group of data cards (custom data objects) associated with a contact and perform various operations, writing the result to a field in your contact record. One of the operations supported is "latest of date values." You could use this app in Program Builder to scan the purchase dates for every contact, populating a new Most Recent Purchase Date field in your contact record. Once you have run this on your database and populated the new field for every contact, it should then be very simple to create a filter to look for records with Most Recent Purchase Date not within the last 75 days. (You'd also want to add this app to your CRM integration program so that the Most Recent Purchase Date field was always kept current when you imported new purchase records.)
The page describing the app is here: Data Card Evaluator. Joel Rothman, a product manager with Oracle, provided the list of supported rules in a comment on that page, and I am guessing he could answer more detailed questions about the app. The write-up does note that you must have Data Card access provisioned in your database, so if you have the Basic trim and have not purchased the Data Tools service option, you may not have access to this capability.
Let us know if this approach works out!
Take care,
Andy