UPDATED: 5 Ultra-Rare Demo Tapes for Super-Collectors
Metallica's 1982 No Life Til Leather and "Sucking My Love" Tapes
But you definitely can't buy the "Sucking My Love" tape, as only one copy apparently even exists, owned by Ron McGovney, Metallica's original bassist. You can listen to the very early recording however, which is a cover of a Diamond Head track that the band never recorded again.
The song was apparently recorded in McGovney's garage, "by suspending a $49.00 ghetto blaster in the air, and with James singing through the guitar amp."