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You may return it IF they have returned your credit and given you an extra credit for the postage.
It is in the Help docs - look under Received with a problem.
At the very bottom of the There Is a Problem with a book I received topic is this:
In all cases of problem swaps, you are not obligated to return the problem item - you may do this if the sender provides postage (eg, a self-addressed, stamped envelope sent by snail mail, or postage by PayPal and the mailing address in a Personal Message).
Last Edited on: 11/1/11 3:05 PM ET - Total times edited: 1
And under the topic "requestor says there is a problem with a book I sent"