[Theohazard]

I agree with everyone else: there's no moral problem here.

Let's pretend he didn't have account issues with Gunbroker and let's also pretend he's not scamming you: You had one item to ship and you shipped it to the address he provided. But because he couldn't update his address even though he moved six months ago, you have to send him another package? You shouldn't eat the cost of a second ammo shipment just because it took him six months to update his address.

Packages with the wrong recipient usually make their way back to the sender, so the if original package makes its way back to you, your only moral obligation is to try to re-send it to his new address (after he pays you a new shipping charge).

I'm heartened by your desire to do the right and moral thing, but I'm also glad you're wary of a possible scam; this whole thing screams "scam" to me.