Currently, they sell demonite ore / crimtane ore only after EoW/BoC is down.
Basically...
start: currupt/crimson blocks, wood, basic drops and stuff (like rotten chunk, vertebae, mushrooms, etc.)
EoC: thrown powder, biome exclusive fishes, summon item for EoW/BoC, and some other stuff
EoW/BoC: ore and other material (shadow scale/tissue sample), shadow orb/crimson heart drops and some other stuff
Queen Bee: fountains
Skeletron: thrown waters
hardmode: hardmode material (cursed flames/ichor), hardmode exclusive fish, crafting stations (crimson only)
1 mech boss: greater mimics drops, solutions