I would contact a lawyer. This is why most used equipment is paid in cash or certified bank check. American Express is notorious for only worrying about the card holder.
If you have everything you said then you should be able to get your money or your equipment (I would only accept the money at this point) using the courts. I would also consider contacting the local police where you are. The amount of money will decide how serious this is.
None of us are lawyers, all though I do eat lunch with mine twice a month.
Who is your credit card payment processor?
They should be fighting this for you.