SQL Query for Max Date

I got this SQL query that I'm running against a table to pick the latest date an item was sold (PurchaseDate), pick the customer that bought it along with other info, the thing is, an item can be returned by one customer and bought by another, so If I'm running Max date function and grouping on Customer, I'll get both customers, the one that returned the item and the one that bought it, and it's critical to bring the customer out in the query. I need to select the item (Unit_Serial_no) by the last date it was sold (PurchaseDate) by that customer, and ignore past customers that had that item as some point. Please advise.
Regards