Is it legal to sell stamps for more than value?

These days postal customers have a multiplicity of options for buying postage: They can purchase stamps not only at post offices but also from private mailbox chain outlets and from many grocery, warehouse, and convenience stores; they can order stamps by phone, through the mail, or over the Internet; and they can even print out postage stamps themselves through programs operated by authorized U.S. Postal Service (USPS) vendors. It wasn’t all that long ago, however, when many customers had few (if any) alternatives for obtaining stamps other than by trekking down to the local post office. It might be memories from those days — when the U.S. Post Office Department was strictly a government agency (i.e., prior to the creation of the USPS in 1970) and its offices were the only sales outlet for stamps for many people — that have fostered the mistaken belief it is illegal to re-sell U.S. postage stamps for a price greater than their face value.U.S. postage stamps may not be resold for a price greater than their face value.

What the heck don’t understand