This is not necessarily fraud. Cargo Largo specializes in re-selling lost shipments. Their whole business model is buying lost UPS / Fedex / etc packages at auction for pennies on the dollar and re-selling those items.
I suspect that they are obligated to give you back your amp, if you can prove you are the rightful owner. Before going to the police (some folks here are real trigger happy), contact Cargo Largo directly. If I were a betting man, I expect you two can work it out.