If I did that, I would try to contact the person to give back their item, but that's just me o:
There's nothing "right" about what he "should" do ... it's ultimately up to him.
In one light, it IS the seller's fault. He didn't double check his store, it all falls down to his loss.
In another, you could buy it, and pass it back to the owner later. That's a really nice thing to do.
Or, you can buy it. Someone's going to snatch it up, are you going to take it or not?