This doesn't sound right. Even if two different sellers are selling the exact same brand manufactured product (say, an Android Phone), via FBA, when a customer

This doesn't sound right. Even if two different sellers are selling the exact same brand manufactured product (say, an Android Phone), via FBA, when a customer buys from seller1, they should get the Item1 which was actually supplied to FBA by Seller1. They cannot get Item2 which was supplied by Seller2. When the customer returns the Item1 for manufacturing defect etc reasons, it has to go back to the Seller1. This is because Item1 has "manufacturing date and batch number", may have "imported from" etc that are different from Item2 by Seller2. Seller1 may have gotten it with different pricing terms etc.