This is a
term of art among
music traders. Commonly referred to as a "
B&P", one party offers a
blanks and
postage deal to another, usually via a
usenet group or
mailing list. See
etree.org for an example of a mailing list on which one can find B&P offers.
The exchange works as follows: the party who is "getting" the B&P sends the one offering the B&P blank discs in a number sufficient for the concert in question, and a self-addressed, stamped envelope. The offeror burns the discs for the lucky guy and sends them back. Detailed instructions can be found at http://www.mcnichol.com/bnp/, but that is the jist of it.
This type of arrangement is generally in keeping with the taping and trading policies of bands like Phish and the Grateful Dead. They allow trading of recordings of their shows, so long as no money changes hands.