US stamps are not designated legal tender. The smaller US coins are legal tender only up to a given amount. Merchants don't have to take your bag of pennies as payment.

However, all current US currency is legal tender. If offered in payment of an obligation and refused, the obligation is discharged. Of course, you need a witness to this.

The business of not taking larger bills is mostly a question of presenting a large bill for a small transaction and expecting change. I don't see how a merchant could get around the law by posting that he won't accept large bills.... but he could refuse to return change, or he could refuse your business entirely. If he sells you something, he has to accept payment in legal tender.

The legal tender laws came about to make currency acceptable as an alternative to real money.... gold and silver. The government forces you to take the paper or the valuless token coinage as a way of facilitating trade. It also facilitates the inflation of the country's currency which is always to the government's benefit.