The original Thai report is clear on this.
The old radiotherapy head was was "scanvenged" (aka "stolen", it was on private property and they did not have permission to take it) by 3 guys, from an unsecured site where it was being stored. They then dismantled it(partially at a home, and partially at the junkyard as I recall) and sold anything of value (ie metal) to a scrap merchant. The 3 guys involved in the 'theft' survived, with various levels of radiation burns, as their time exposure was less than the junkyard workers.
The "sale" in the reference you mention refers to the fact that the junkyard bought it from the scavengers. The "accidental" sale refers to the fact the that thieves had no clue as to what they stolen when they sold it to the junkyard.
Oh, and it was the thieves dismantled the head, not the junkyard workers.
From: JulioLS