If it is the one from Cargo Largo, I would contact the police in Independence, Missouri USA. Ask for their Detective division.
Tell them they are selling your amp and you have paperwork to prove it.
Cargo Largo resells equipment on consignment so your amp probably is not there. Tell the police you would expect Cargo Largo to purchase the amp from the person selling it and ship it to you, since Cargo Largo is selling a stolen item.
Cargo Largo contact info to give to police
Cargo Largo
Phone 1-816-350-6286
7460 Stadium Drive,
Kansas City, MO 64129
USA
Kansas City, MO police
Kansas City Missouri Police Department Headquarters
1125 Locust St
Kansas City, MO 64106
USA
phone 1-816-234-5000