I might note that one of the qualified facilitators on the list recently "sold" me a block where the original entity which obtained it in the 1990s was still announcing it to all of their peers and trantsi after the wire transfer had been done, the ARIN process was done/ticket closed, and the block resided with my AS.
It took a significant amount of badgering the original block holder (an entity with which we had no pre-existing relationship or direct contacts into their engineering department) to get them to withdraw the announcement, which we did independently of the broker and quicker than they responded to us. So my message would be to do your own due diligence and investigation of IP space and don't trust what the "broker" tells you.