What happens if you don't return exchanges on Amazon?

If you don't return the item, they will charge you for the original and the replacement. 

The first time it happens they might not do anything.  If it happens again they'll charge you for the items you didn't return.  Eventually they'll close your account and put you on the list of people they're never going to accept as a customer again with all the other people who have tried to screw them over like this.  

This has NOTHING to do with Taxes.

If you want to exchange a product purchased from Amazon, you have to return it.

No return, no credit for an exchange.