What this statute means, in the words of the United States Treasury, is that "[A]ll United States money . . . is a valid and legal offer of payment for debts when tendered to a creditor. There is, however, no Federal law mandating that a person or organization must accept currency or coins as for payment for goods and/or services."
That's it. All this means is that the Federal Reserve System must honor U.S. currency and coins, not necessarily anyone else. U.S. currency and coins can be used for making payments, but a debtor does not have to pay in legal tender, nor does a creditor have to accept legal tender. A creditor can accept all U.S. currency and coins as payment, he can accept no U.S. currency or coins as payment, or he can pick and choose which currency and coins he will accept. As a creditor, you can accept checks, credit cards, and money orders only if you so choose. If you don't want to accept currency in denominations higher than $20, you're perfectly entitled to do so (and plenty of establishments have such a policy). If you want to decline more than 25 pennies as payment, you're allowed. If you don't want to be paid in anything other than nickels, you can specify that. If you make a deal to trade your '57 Chevy for six elephants, you don't have to accept cash in lieu of the pachyderms. Likewise, the other party in the transaction can offer whatever he wants as payment, as long as you're willing to accept it. No law says I can't offer you 500 comic books as payment for your boat, just as no law requires you to accept 500 comic books as payment for your boat.
As a general legal principle, however, it is assumed that payment will be made in general legal tender unless otherwise stated. If you won't accept anything larger than a $20 bill in your store, you'd best state that up front or post a sign to that effect. If you agree to sell your '57 Chevy for $8,000, you can't later claim that you really wanted $8,000 worth of elephants. If you advertise bananas for $2 a bunch but want to be paid only in dimes, you should include that requirement in the advertisement to avoid any legal complications. Likewise, if I agree to buy your pinball machine for $500, I can't later insist that you accept $500 worth of Italian gold coins for it.