When a Villager loses/changes their profession their previous trades for that profession aren't remembered.  For example if you have a (not traded with) farmer villager with two trades, removing the compost block he is connected with and placing it back again will change his trades to potentially different ones whether intended or not.  This also extends to Enchanted books, which can potentially allow a player to pick and choose what book they want to trade for if exploited properly for an unfair advantage.