I've tried A LOT of different syntaxes and nothing has allowed me to reach my goal. I want to create a Villager that A) sells spawners (I've been able to do this) and B) have those spawners spawn mobs other than pigs. So like a zombie or something. From there I could name him a "Zombie Spawner Trader" or something like that. Keep in mind, my villager is being created no problem and he's buying and selling things according to what I've programmed, all that's wrong is that the spawner is only spawning pigs. I have tried NBT tags: "Entity", "EntityId" and "SpawnData" with their proper syntaxes according to the wiki, and none of them worked. It proceeded to give me a pig spawner as usual.
Some code:
/summon villager ~ ~1 ~ {CustomName:"Zombie Spawner Merchant",CustomNameVisible:1b,NoAI:1b,Invulnerable:1b,Offers:{Recipes:[{buy:{id:"minecraft:gold_nugget",tag:{display:{Name:"Gold Coin",Lore:["$1"]}},Count:32},sell:{id:"minecraft:mob_spawner",Count:15,tag:{SpawnData:{id:"minecraft:zombie"}}}]}}