Minecraft wiki entry for Things_not_to_do, associated text: Lava buckets: Armorers buy a lava bucket for an emerald. However, lava is not renewable even in the Nether and buying a bucket from a wandering trader costs 5 emeralds. Also, a bucket is worth more than an emerald. However, as of 1.17, lava is renewable via pointed dripstone and there is a trade loop with the fisherman (emerald to bucket of cod, raw cod to emerald) that lets a player obtain free buckets, making this a good trade (1.17+).
Diamonds: Blacksmith villagers (the weaponsmith villagers and the toolsmith villagers) buy a diamond for an emerald. However, a diamond is worth a lot more than an emerald. On top of that, it is hard to obtain and not renewable.
Wheat and Beetroot: Farmers buy it for emerald. However, using fortune on these crops yields only 1 wheat or 1 beetroot respectively. Also, wheat is more useful for breeding animals and making bread and hay bales. However, if you have excess crops, you can use this trade to get a few extra emeralds.
Coal and Iron: Armorers buy 15 coal for an emerald, and 4 iron for an emerald. Until you have a wither skeleton farm and an iron farm, these items are not good for trading, especially in the early game when you do not have a lot of these resources. The coal can be easy to find underground since it's common, which the butcher, fisherman, weaponsmith, and the toolsmith villager also buys for emeralds.
Gold: Clerics buy 4 gold ingots for an emerald. However, gold is hard to obtain in large quantities. 3 gold ingots are worth much more than an emerald. However, if you have a large gold farm, such as a bastion piglin-killing farm or a zombified piglin farm, this is not a big problem.