Complex sentence: In the mid 90's discovering among his collection was one of Marilyn Manson and the Spooky Kids first self made promo tapes, disputed by some fans as never existing, the Beaver Meat Cleaver Beat, shortly thereafter was sold to a collector from Japan.
Split into simpler sentences: In the mid 90's, one of Marilyn Manson and the Spooky Kids first self made promo tapes was discovered among his collection. The Beaver Meat Cleaver Beat was shortly thereafter was sold to a collector from Japan.