"Buyer scam"

"Has anyone ever dealt with this issue?"

Yes, plenty of new sellers who don't know the pitfalls of sending expensive items overseas.

Sellers rarely win an item not as described case.

"The agent was understanding and knew US customs policies so said you just need to ask eBay to step in- don’t do anything else and our expert team will look and help"

Seriously ?

That's what they would have told the buyer.

The seller  must  supply tracked return postage in an item not as described case and because you didn't, 

eBay stepped in on behalf of the buyer, refunded in full and closed the case.