Typically, the following things are needed.
1. The slayer has to have a current need, with little artifact competition.
For example, right now, the Ophidian slayer is a hot commodity, due to that event. Reponds, though they help you on the other side of the fight, do not, simply because they are too common (Soul Seeker, Night's Kiss, and Blackthorn's Kryss replicas)
Also popular are Dragon Slayers, as a good Dragon slayer eclipses most reptile slayers for actually fighting dragonkind. Navery has made Spider Slayers popular, and Occasionally, you might sell a scorpion slayer to someone who farms Miasma.
2. Any additional properties better fit well.
Most people won't want hit area effects, unless they are those that do the lower level of champ spawns (and those stack really nicely on radiants, with their whirlwind special). Most people, though, are looking for leeches (mana for Sampires, life for Whammies, are popular).
3. Some slayers will never sell well.
Essentially,
Elementals: forget about selling any of the elemental minor slayers (with the possible exception of earth ele slayers to a miner, and maybe a blood elemental slayer to someone who camps in Blood dungeon).
Repond: Orc slayers only sell during an orc invasion, and Ogre slayers have ebbed in popularity with the Spellweavers making the Arctic OL spawn unreliable from occasional failures of respawn after a Dryad Allure. Troll slayers have always been garbage, and the only skill without a good artifact Repond slayer is Mace Fighting.
Undead: Lots of artifacts do it, so typically only someone looking for a high-end custom weapon will want a crafted undead slayer.
Arachnid: See #1. not many people do Mephitis, or Terethan Keep, so the super slayer and Terethans don't sell well at the moment.
Reptile: Also, see #1. Snake slayers are only good for fishermen, Yamandon farmers (which aren't that common anymore) and people who like doing the Sleeping Dragon spawn (for the boss). General reptile slayers aren't that popular, except for spawning.
Demon: demon slayers sell okay, but since Gargoyle slayers don't work in PvP, and few actually kill what gargoyles are in game, they are a waste.