Who is responsible for my order? What should I do?

I ordered a package through Amazon from a third party seller on May 18th. Although the site says that the package was delivered by USPS (-_-) on May 22nd I never received the package. I did not sign anything so there is no receipt available and I believe they just dropped the package off downstairs in the vestibule or who knows where. So I was just wandering how I should go about this. The third party seller tried to tell me they are no longer responsible so I need to talk to USPS. I have talked to USPS numerous times and they are really frustrating to deal with and I have gotten no where with them. Anyone had this problem before? Any suggestions?