Have you talked with UPS? They have GPS recordings of where things were dropped off. I had a case where UPS said an item was delivered but it was not here. They checked their GPS and found they had delivered it to the incorrect address. They picked it up and had it to me the next day.

If that doesn't work, then contact the seller. Explain the situation. Typically they are responsible for delivering the item to YOU. So if it was signed by the incorrect person, they should replace the item.