If there are 3 SalesID's with 1 item in common but the overlap is for only 2 of the SalesID's, does that need to be reported or does the over lap have to occur for all SlaesID with that item?

In your sample data look at SaleID 2,3, and 4 and ItemID 5. The SaleID 2 and 3 overlap each other but 4 doesn't.