I'm confused. Did you get the sale price + $1000 or did he send you $1000 and ask for you to return the difference? Have you actually been able to transfer money to your bank account or not yet? When I receive money in Venmo I'm able to withdraw the funds immediately.

Regardless, I think you would be SOL on AMEX assisting you on this with the facts as stated. Your recourse would be to file a police report and then sue the person in small claims court, which I'm sure you won't want to/be able to do.