Try the Commands Troubleshooting and Help page if you get stuff with server errors.
Minecraft summon wandering trader command
Using the summon command to spawn in a minecraft wandering trader is simple, take the command and enter into chat and presto you have a wandering trader. Options for setting trades that villager can make.
Wandering Trader trades
Building trade data is complicated and takes some time. Each trade needs at least one buy item and one sell item. You can add nbt tag data for named and enchanted items (other tags may work too). Summon generally creates a villager in a blank state, so it's best to set all the options that you would like. Remember long commands need to into command block to get around the chat limit.
In this example you can buy a iron sword from the villager
/summon minecraft:wandering_trader ~ ~ ~ {VillagerData:{type:plains,profession:farmer,level:2},Offers:{Recipes:[{buy:{id:emerald,Count:1},sell:{id:iron_sword,Count:1}}]}}
In this example you can buy a named iron sword from the villager, Note the
/summon minecraft:wandering_trader ~ ~ ~ {VillagerData:{type:plains,profession:farmer,level:2},Offers:{Recipes:[{buy:{id:emerald,count:1},sell:{id:iron_sword,count:1,components:{custom_name:'["",{"text":"Pointy","italic":false}]'}}}]}} /summon minecraft:wandering_trader ~ ~ ~ {VillagerData:{type:plains,profession:farmer,level:2},Offers:{Recipes:[{buy:{id:emerald,Count:1},sell:{id:iron_sword,Count:1,tag:{display:{Name:"[{\"text\":\"Pointy\",\"italic\":false}]"}}}}]}}
In this example you can buy an enchanted named iron sword from the a villager
summon minecraft:villager ~ ~ ~ {VillagerData:{type:plains,profession:farmer,level:2},Offers:{Recipes:[{buy:{id:emerald,count:1},sell:{id:iron_sword,count:1,components:{custom_name:'["",{"text":"Pointy","italic":false}]',enchantments:{levels:{sharpness:1}}}}}]}} summon minecraft:wandering_trader ~ ~ ~ {VillagerData:{type:plains,profession:farmer,level:2},Offers:{Recipes:[{buy:{id:emerald,Count:1},sell:{id:iron_sword,Count:1,tag:{display:{Name:"[{\"text\":\"Pointy\",\"italic\":false}]"},Enchantments:[{id:sharpness,lvl:3}]}}}]}}
What about Villagers, and Zombie Villagers
Try the full villager mob generator, which has a easy select of types, professions and trades
Or if you just want the zombie villager, which details options for that mob.
Suggestions or found a bug
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If you're looking for a particular summon mob generator you can link to these specific versions of the command generator.