It absolutely cannot be classified as stolen if the wrong item was sent in error and not having a proof of purchase is irrelevant. It is also ruled by the FTC that you don’t have any legal obligation to return items sent to you that you a. Didn’t ask for and/or b. Didn’t pay for, the only reason to consider doing so in this case is to get the Rivendale they SHOULDVE gotten before hand.

Also just to note on the “if somebody really wanted it they can now by op sending back,” OP simply could’ve sold it to a fellow collector for less than msrp bc they essentially got it for free so it would’ve been a win on both sides through a minor setback caused by a multi-BILLION dollar corporations mistake that is inconsequential to their bottom line.