Not quite. "Selling you something" and "satisfying a debt owed" are not the same thing. A sales transaction is not the same thing as a debt.
It is perfectly legal for a counterparty to decline to enter into a sales transaction with you because they don't like your proposed payment terms. They are not required to accept cash (or anything else that they don't like.) That is not satisfaction of a debt owed; no debt has been created, because they refuse to give you anything in the first place.
The "legal tender for all debts" part means that _if you already owe someone money_ for some reason, and you offer them cash, they cannot refuse the cash and then pursue legal action against you for not paying the debt. The court must rule that your offer of cash is sufficient to satisfy the debt.