www.federalreserve.gov/faqs/currency_12772.htm

Is it legal for a business in the United States to refuse cash as a form of payment?

There is no federal statute mandating that a private business, a person, or an organization must accept currency or coins as payment for goods or services. Private businesses are free to develop their own policies on whether to accept cash unless there is a state law that says otherwise.

A few states have introduced bills to require taking cash (Idaho, Mississippi and North Dakota), but as far as im aware none have ever actually passed into law.