I suppose that the author also has major difficulty with a "deed" identifying the transfer of land, since the land isn't actually going anywhere.
"Used" MP3s don't refer at all to the data - they refer to *the right to use the data*, the license to the recording. What is being transferred is the right to own and listen to it. It's the same basis on which libraries lend audio CDs, which are easily rippable: the point is not the physical access to the data, but whether or not the individual has the right to use it.
There's a ton of value in the "used" MP3 market. Imagine that you have an entire licensed collection of music from an artist - say, The Beatles' entire discography - and you either need some cash or you don't like them any more. Rather than someone buying the MP3s from iTunes (which actually isn't "buying" the music - most of the cost is in buying *a license* to the music), you could "sell your used MP3s" by transferring your ownership (and, incidentally, the data) to someone else.
As for enforcement: While it's impossible (and terrible policy) to enforce MP3 licenses against individual consumers, lots of other places *need* licenses for music, and the ability to demonstrate valid ownership and rights - like DJs and dance halls. The license itself has significant value, apart from the data.
In summary, a "used" MP3 market would create an actual free market for music licenses, rather than the controlled, price-inflated, supply-constrained oligopoly that we have now - which is the main reason that the MPAA opposes it. It's also why we should all encourage its development.
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