It's super easy to do. Use a VPN on the machine that connects to the exchange. Open an account on a exchange banning USA citizens, use fake info. Dump money into said exchange via prepaid disposable cards paid in cash. There are also others ways to transfer money into the exchange.
Do your thing on the exchange.
Cash out by transferring to a wallet and then there are several avenues that come to mind to transfer btc to fiat. Bitcoinlocal.com comes to mind, so does a foreign bank account.
I don't condone this type of behavior, just wanted to show you how easy it is.