How To Select A Specific Custom Enchantment In A Villager Trade?

You can lock a loot table, (and presumably a villager trade, as these are a special variety of loot table), and add additional loot functions to add custom data to items in the table. (Including damage, potion effects, and specific enchantments.) Once you lock the table, you should find each entry in the table has a 'functions' section, with the only provided function being to 'set count,' randomizing the number of items based on your parameters.

Here's some common loot table functions you might find helpful: (Add a comma after the first function, and paste these into the functions section:

Enchant an item with random levels using the provided enchantments. You can list as many enchantments as you like, (you can find the correct registry names on the wiki, they're mostly self-explanatory); but just make sure the last one doesn't have a comma after it.

{
  "function": "enchant_randomly",
              "enchantments": [
                "power",
                "mending",
                "knockback"
              ]
}

Damage an item with durability within a set range: (In this case, between 40-70% durability remaining):

{
    "function": "set_damage",
        		"damage":{
        		"min":0.4,
        		"max":0.7
        		}
}

Set the potion value of a splash potion, bottle, lingering potion, or tipped arrow. (Again, registry names are important, but generally straightforward):

{
  "function": "set_nbt",
              "tag": "{Potion:\"minecraft:weakness\"}"
}

...Etc. Just make sure things are spelled correctly; loot tables with errors won't prevent the game from compiling, but any error in the loot table will prevent the entire thing from loading.

It's possible that villager trades use different code than loot tables? There's also a Microsoft Guide that explains loot table functions better than I can. (Though a couple of them are bedrock specific, most work for java modding.)

The trouble is I haven't actually made a villager trade with enchantments; I've only ever done this with loot tables, and, looking into it, villager trades are quite a bit different. 

The principle should still be the same- you would need some sort of loot function, (the functions may be different for villager trades), that you would put in the code to enchant the traded item, and you would need to reference the enchantment using your mod's id instead of minecraft, which would likely also be kind of complicated. Possible, but I'm not personally sure how you would do this.