Return Package due to invalid shipping address

[harborgifts]

I had sold an item which was to be shipped to the buyer.
I reviewed the shipping address provided by Ebay, put it on the box, mailed it out and never gave it a second thought.
 
It wasn't until I checked on the tracking updates that I see that the package couldn't be delivered to the address, and is being returned to me.
I emailed the buyer the day of purchase, thanking them for their business, the day I shipped their package to inform them their order is on the way, and the day I found
out the the package was being returned. It wasn't until I heard from the buyer the day that the package was being returned.
Upon reading the buyers message, everything was correct with the exception of an apartment number.
I went back and double checked the information that Ebay provided, and there is no apartment number listed at all.

 So now the question is who's suppose to pay for the package to be sent again?
Considering I shipped to the address provided by Ebay, I can't see how I'd be responsible to pay for shipping twice.