Trading Behavior

Making an entity a trader is accomplished via minecraft:trade_table or minecraft:economy_trade_table components. Both of them will open a trading UI from the given path, but the economy trades component has some more options referring to some Village and Pillage trading mechanics. Other AI goals you'll need are minecraft:behavior.trade_with_player, optionally minecraft.behavior:trade_interest (allows the mob to hold/offer an item) and, potentially, "minecraft:trade_resupply": {}.

For a simple trading UI, trade_table + trade_with_player components should do the trick.

1. Add "minecraft:behavior.trade_with_player": {} to your entity's components.
2. Copy the following code into a component group of your entity. I'll call mine "wiki:trader";

"minecraft:trade_table": {
	"display_name": "Trading Entity", // Text to be displayed.
	"table": "trading/trading_entity_trades.json", // Path to the trade table file
	"new_screen": true //If set to false, the UI will display as the pre-Village&Pillage one.
}

3. Now make sure the component group is added to the entity via an event. It's a good idea to add it in minecraft:entity_spawned event, since it triggers on spawn. If you don't feel confident with events and component groups, make sure you're familiar with entity definition rules/concepts. See the Intro to Entities.