If I purchased a stick of 32GB DDR4 RAM from you for $100, am I now not in $100 debt to you? We are private parties - can you refuse to accept a legal $100 and force me into paying you by Zelle?

Yes, but the argument is that the seller is under no obligation to sell you the stick of RAM originally that will incur the debt if you offer cash instead of Zelle. If you did manage to receive the stick of RAM without paying for it during the initial exchange and so still owe the $100, then yes, the seller is required to accept cash to settle that debt.