Minecraft wiki entry for Villager_(old), associated text: After trading a new offer once, the villager will allow a new tier of offers. After 2-12 times an offer is repeated, the villager will lock the trade offer. That is, the villager will no longer offer this trade. When this happens, the player will have to use another new trade offer in the villager's window once (or several times if it is already used once), and then wait for a short time. If green particles appear, all trades unlock. That is, the villager will start offering all trades. There is a maximum number of tiers each villager can possess, varying by career. Once the villager has unlocked all tiers, it will not open any new ones. However, players will still be able to renew all offers by trading.