go to mc stacker and use the villager summoning generator, or watch Xisumavoids tutorial here

www.youtube.com/watch?v=ptf0L5rPoqU

The best way to solve that problem, in my opinion, is the Minecraft wiki: the villager page. It has a data value section which is basically an overview of properties which you need to specify to make this work, pay special attention to the Offers:{}, that's what you need.

So let's do something simple as example: a villager which will trade my head for 3 diamond blocks.

First we need to determine the villagers Profession, Career and the level. Not only does this determine the looks, it can also influence the trades which are automatically generated. I'll just use a butcher for that extra "scary" effect and set his level to an amount where it won't generate more trades. You will notice that I split up the command, I do that to make it easier on myself and keep track of all the opening and closing sections:


/summon minecraft:villager ~-3 ~ ~ {
CustomName:"\"Scary villager\"",Profession:4,Career:1,CareerLevel:4,Offers:{Recipes:[
{rewardExp:1,maxUses:9999,
buy:{id:"minecraft:player_head",Count:1b,tag:{SkullOwner:"ShelLuser"}},
sell:{id:"minecraft:diamond_block",Count:1b}
}
]}}

And if you want to add a new trade you simply add another {} section behind the single closing curly bracket.

Hope this can help.