Minecraft wiki entry for Experience_farming, associated text: Farmers will each buy one of the base crops immediately, and as they level up they will all buy pumpkins and melons. The crops can be automatically or easily farmed, especially the pumpkins and melons.
Wood can be converted to sticks and sold to fletchers. Later, fletchers will buy string and feathers.
Paper can be sold to all cartographers and some librarians. If leather is plentiful, the librarians may also buy books. This will also help level the librarians to sell more enchanted books.
Shepherds will buy one color of wool up front, and later they will buy some dyes. Some luck is needed with the dye trades, to get dyes that you can farm easily (that is, no lapis lazuli or small flowers). Bone meal is key here; it is easily produced with a composter (and/or skeleton bones), and besides crafting dyes directly, it can be used to rapidly grow the tall flowers.
Apprentice: white (bone meal); black (squid ink); gray (black+white); lime (green+white);
Journeyman: red ( rose bush or beetroot); yellow (sunflower); pink (peony or red+white); light gray (black+white); orange (yellow+red)
Expert: green (cactus); brown (cocoa beans) magenta (lilac)
The same cleric who sells you the bottles will buy rotten flesh from a mob farm, and later netherwart. If you have a gold farm, that can also be helpful.
If you have an iron farm, you can sell to the various blacksmith types, rapidly leveling them up to selling you diamond gear.