I actually took a villager and transformed him into the zombie villager he was a librarian i cured him using golden apple then i kept him in a room with 12 other villagers then after few minutes the librarian i cured and few others who where not villager zombie or  cured where giving me cheap prices i know only the cured villager should give me that price but few others also started giving me the goods  for cheap although they where normal and were never turned into a zombie villager or cured