Separate issues here -- one is mail fraud and one is altering a label such that the intended recipient cannot be identified.
Let's start with "refused and return to sender." The mail carrier was able to deliver the envelope on May 10 because the street address exists. For a multi-unit building -- especially one with a management office -- specific names do not matter and "somebody" is there to receive mail and decide if the tenant is still there.
In this case, the recipient's name was not on the label, nor was the unit number. Therefore, the management office could not identify a tenant and correctly refused the mail the next day (May 11). The piece is now en route back to the sender (presumably the OP's buyer). However, if the sender removed his/her return address, the piece may end up in the dead letter office. Since the label is an official prepaid return label, it's remotely possible the piece may end up at the originating PO (zip code is printed on the label). They may be able to find out who actually sent the piece (OP's buyer) or who issued the label (OP or eBay). After all, a Paypal account provides plenty of clues!
The OP should be in contact with both the buyer's PO and his/her own PO in case the envelope arrives at either location. Certainly, both eBay and Paypal should be involved, too, because of the obvious mail fraud -- returning an envelope with unknown contents vs. an 8-lb. box of merchandise.
Bottom line: The OP did not "refuse" the mailpiece. It was deemed "undeliverable as addressed" -- no name or unit number -- and properly given back to USPS.
~~C~~
My Glass Duchess
Quoting Mom: In polite society, "hey" is for horses.
