Have at least 50 silver coins total in your inventory, piggy bank, safe, or defender’s forge
Make sure there is an empty house available for the Merchant
Wait for the Merchant to move in during daytime
Buy the Merchant’s items from his shop interface
If you mean a specific item sold by the Merchant, check his shop after meeting his spawning requirements
If you mean the Merchant NPC itself, meet the coin requirement and housing requirement to spawn him