Firstly - the following discussion has nothing to do with promo items such as "Disk Union" boxes etc. While it's true they do sometimes include a bonus CD, these are similar to "Promo obis" (ie. a replica of an older release - see the FAQ on promo obis).

So.... A promo CD is "given out" to radio stations, journalists etc in order to promote an artist's work. They are also known as "cut-outs". In most cases the CD is considered to be leant to the recipient - it remains the property of the manufacturer.

Legally speaking, promo CDs cannot be sold or resold. In order to make it more difficult to sell, the CD case (and sometimes the CD itself) is usually marked or permanently damaged in some way.

Despite the legal implications, and the obvious violation of eBay's policies, promo CDs are regularly sold on eBay. That said, eBay does occasionally cancel auctions of promo CDs. For example, robots monitor eBay listings on some sites and report promo CDs to the manufacturers or copyright or other groups representing artists' and corporate interests and the auction will be cancelled at the request of the interested party.

The types of marking found on CDs vary dramatically from a simple removable sticker (which often causes no damage) through to a saw cut through the spine of the case.

Following are examples:

Saw cut through the spine. This was the standard way of marking promos in the days of vinyl (pre bar codes in other words) and is still practiced today.
Drill hole or slashed cut through the bar code (and therefore the case too in the case of jewel cases).
Sticker on the bar code.
Etched stamp on the CD itself.