Ebay problem as a seller

[pherbener]

I know this specific item is not prewar but I'm hoping it can stay here as I deal mainly in prewar stuff and this is more of an ebay question.

I just got a message from a buyer who had already left me positive feedback for a transaction of a 1985 Sealed Topps Baseball Set. Here's the item in question.

https://www.ebay.com/itm/15384195582...84.m1561.l2649

He now messaged me and says he now has opened the sealed set and it contains a 1986 set and asked me what I'm going to do about it.
A- I could have never known it was an 86 set if in fact it was actually in the box and it wasn't changed.
B- It's no longer sealed so I can't get back what I sold
C-I did my due diligence and found the exact same box set sold and still selling on ebay as 1985 sets(why would I have assumed anything else?)

Examples:

https://www.ebay.com/itm/FACTORY-SEA...AAAOSwETxeXcyj
https://www.ebay.com/itm/1985-Topps-...MAAOSwD2Bc3H-U

Am I the a$$hole here thinking I shouldn't be responsible for this?