sometimes if you trade a villager and he gives you a discount, instead of lowering the price he increased it (in my case from 36 emeralds for a bell to 32, and after I bought it again he increased it to 64) I was in creative, and bought the bell again and again in high speed (even after he restocked it along with other things, I think it might affect survival as well and might happen with other stuff that you really need to buy