While I’m not an attorney, the website you describe would be considered a “money transmitter business” (MTB) which, in the US, falls under the jurisdiction of The Dept. Of Treasury FinCEN (Finacial Crimes Enforcement Network) which has very strict regulations regarding anti-money laundering (AML) activities. You would need to register with FinCEN and comply with their regulations to legally perform such transactions on your website regarding AML/KYC (KYC is Know Your Customer). MTB laws also vary from state to state when it comes to cryptocurrency exchange. Just make sure to consult with a lawyer before you go live to protect yourself.