Skeleton Merchant

He is not a town NPC, he needs no house, and you cannot make him stay. He turns up on his own somewhere in the Cavern layer, and he disappears the moment he goes off screen, so every visit is a chance encounter you either take or lose. He has no map icon, which rules out the usual way of spotting an NPC.

A Hunter Potion and a Lifeform Analyzer turn hunting him from luck into a routine, and the analyzer shows his current name rather than his title. There is a second tell: he suppresses spawns around himself, so a stretch of cave that has gone suspiciously quiet, or a Radar reading far lower than it should be, usually means he is nearby. He also will not appear if you have a Shadow Candle placed or in hand, and he never spawns on jungle grass, on corrupt or crimson blocks, in water, or in front of natural desert or spider nest walls.

Most of what he carries depends on the current moon phase, and the whole shelf rerolls at dawn without him having to despawn and come back first. That makes him a return trip rather than a shop. If the thing you wanted is not there, note the phase and come back on the right night. A handful of lines ignore the moon and are always available, so you can rely on him for explosives and rope no matter when you bump into him.

The yoyo gear is the real reason to hunt him. The Yoyo Glove appears once you are in Hardmode and the Magic String only after a mechanical boss is dead, and neither exists anywhere else in the game. Together they change a yoyo build more than any crafted upgrade will. Bring the money in advance, because the string is expensive and he will be gone before you can go and dig some up.

The Artisan Loaf is the other one to plan around. It is a single purchase per character, permanently extends how far your crafting stations reach, and it stops appearing once you have eaten one. If it is in stock and you can afford it, buy it. The Magic Lantern is worth grabbing for treasure hunting, and the Slap Hand only shows up on a Blood Moon in Hardmode, which makes it easy to miss forever.

For caving, his glowsticks are the best light source in the game for the job, since they light up ore through walls as they fall. The flare version does the same thing but needs a Flare Gun sitting in your inventory before he will offer it.

A large slice of his rotation is ordinary surface chest loot: an Umbrella, a Step Stool, Climbing Claws, an Aglet, a Wooden Boomerang. If you have explored at all you already own these, and they are only worth the gold when the world has been stingy. The counterweights and the roller skates are each one item in several colors picked by the moon, so there is no reason to chase a particular one unless you care what it looks like.

His torch stock is a joke at your expense. Ordinary torches and Bone Torches swap places roughly once a second, so which one you get depends on exactly when you open the menu. Buy in bulk while the one you want is showing.

For spawn purposes the game counts him as a whole group of NPCs, which in most setups shuts down enemy spawns entirely in the area around him. He does not raise critter spawns to compensate the way real town NPCs do. If he drifts into your underground farm, output stops until you either kill him or travel far enough that he vanishes. Note that he is not affected by a Shadow Candle once he has already spawned, so you cannot undo it that way.

He defends himself by throwing bones at anything nearby that is not a skeleton. Skeletons of every kind leave him alone and he leaves them alone, including contact damage, though his own bones can clip one by accident and start a fight. That also means you cannot rely on the Dungeon or Cavern population to remove him for you. Lava, boulders and temple traps will, and so will anything you throw yourself. He is not immune to poison despite being made of bone, and no message announces his death.

He picks a random name from a list packed with references, from Jack Skellington to Grim Fandango to a late night television pitchman. Neither the Portal Gun nor a King Statue can move him. Shimmer turns him into a zombie, which is purely cosmetic and reverses if you do it again, and it is a neat inversion of what Shimmer does to zombies. He throws confetti during a Party despite not being a townsfolk. One warning: if he spawns in front of a detonator, talking to him can push the plunger and kill you both.