Does it look like he has bought/sold/left feedback recently? Maybe he's having personal problems/family emergency/etc, something that could easily make someone forget about cards.
Personally, if I didn't hear from him I would refund the entire amount minus shipping and PayPal/ebay fees. Doesn't seem right to keep both the card and the money, and the money would be the easiest thing to return. Like someone else said, it's whatever you think is morally right or wrong.