They are very quick to refund/replace and I wonder if it’s because they have no way to track an item’s origin to a specific seller so they just treat the refund as a small cost in an otherwise very profitable system for them.

They are quick to refund/replace because most of the items sold are returnable and "replace" is just a second order. Basically you have the ability to self service this. So it's a quick way for a CSR to satisfy a customer without doing anything special.