Have at least 50 silver coins in your inventory
Build a suitable house with walls, light, table, chair, and door
Make sure there is an empty valid house available
Wait for the Merchant to spawn during daytime
If the Merchant is not spawning, keep more than 50 silver on you
Talk to the Merchant once he moves in
Buy the Merchant item from his shop if it is available in Fargo’s mod content
If you mean the Merchant NPC item, use a Bug Net or pickaxe on the placed NPC only if your mod/setup allows it
If you mean a Fargo’s item sold by the Merchant, check his shop after meeting the spawn requirement