Armor

• Resists: Anything with at least 30 total resists can be enhanced to better-than-GM. I enhance 30-Resist pieces to 43 and sell them on my vendor for about 1000 gold. The more resist, the higher you can enhance, of course.
• Lower Reagent Cost (LRC): This is really more useful for mages and necros (or paladins in training) so they don't have to carry around reagents anymore. I can sell 12% LRC for about 1000 gold; by the time you get up to 16% LRC, I can get about 10,000 gold. 20% is the max and would go for over 100,000. This is only for armor you can meditate in ("meddable" armor") like leather and "mage" armor.
• Lower Mana Cost (LMC): This seems to be getting more popular, as it allows mages to cast more spells before running out of mana. This ranges from 1-8, but only 6-8 LMC is in demand. Paladins, too, like this, so it's not limited to mage armor.
• Luck: Ranges from 0-100 in chests, and with gold you can add 40 Luck. Keep anything 60 Luck and higher to enhance to 100.
• Mana Regeneration (MR): Not as popular as LMC items, but it allows mana to return faster. Maximum is MR+2, but if you build a suit totally MR+10, that's like having GM Meditation. I had a suit with a lot of MR on it and swapped it out for higher resists, and my slower mana is noticeable.

Weapons

• Damage Increase (DI): Ranges from 1-50. More is better, of course, but this is sort of overrated and usually only adds a couple of points of damage. Still, DI in the 40's are in demand.
• Faster Casting (FC): You can get FC+1 on weapons that are really beneficial to paladins. FC+1 by itself, though, is not in demand, it's just a bonus.
• Hit Area Effect: This was a terrible addition to a weapon. It works well if you're surrounded by a lot of low-level creatures; it's kind of fun taking out a dozen slimes with a single swing. But if there are any high-level creatures around, this is they all get damaged and turn on you (and in pvp, all the greys get flagged on you). Think of it as a "Please Gank Me" modifier.
• Hit Damage (spell): Ranges from 2-50. This is better on faster weapons that hit a lot, as opposed to something slow like a halberd. Lightning is the best spell, but fireball is good, and harm if not on an archery weapon. Magic arrow is useless. Find a dagger with 50% lightning and you'll be rich.
• Hit Life/Mana/Stamina Leech: Ranges from 2-50. Best if combined with something else, these are nice to add on, but since leech by itself doesn't add any damage, it ought to be combined with other stuff. Hit Mana Leech (HML) is best because it'll refresh your mana, and Hit Life Leech (HLL) adds to your health. Hit Stamina Leech (HSL) isn't as interesting.
• Hit Lower Attack / Hit Lower Defense (HLA / HLD): Ranges from 2-50. This is more popular on pvp weapons if combined. HLD is preferred, as it makes them easier to hit. HLA only effects melee, but it's good in combo with HLD.
• Hit Slayer: Super slayers are best (Repond / Undead / Elemental / Demon / Reptile). Some specific slayers are good, but they're become fairly common now (blood elemental / earth elemental / daemon / spider / dragon). Others are completely useless (snake / fire elemental / water elemental / lizardman / troll)
• Spell Channeling: A good damage weapon with spell channeling is useful for tank mages. If it has a FC-1 penalty on it, though, that's the kiss of death. Throw it away or smelt it.
• Stamina Increase: Ranges from 6-8. Some players are experimenting with High Stamina Increase pieces with at least SI+6. If you stack enough of these pieces together, you end up with a dexterity over 170 and swing incredibly fast. Individual pieces don't sell well, but put together a full suit and it'll sell for quite a bit.
• Swing Speed Increase (SSI): Ranges from 5-30. Like the Hit Spell, this is really only useful on weapons that are fast already. A Halberd with SSI isn't much more effective than one without, but a dagger or katana with high SSI means it hits frequently.
• 100% Elemental Damage: Neon weapons with 100% of the appropriate elemental damage; i.e. pink weapons with 100% energy damage, green weapons with 100% poison damage. (Note that a neon blue weapon with 80% cold and 20% energy can be enhanced with shadow ingots to make it 100% cold.

Jewelry

• Damage Increase (DI): Ranges from 1-25. More is better, of course, but this is sort of overrated and usually only adds a couple of points of damage. Still, DI above 15 is in demand. It's more effective if you stack a lot of DI items together with armor.
• Defense Chance Increase: Ranges from 1-15. Makes you harder to hit; DCI+12 or more is worthwhile.
• Faster Casting (FC): FC+1 combined with FCR is in high demand, especially when combined with other mage attributes like SDI. FC1/FCR1 doesn't seem to sell anymore, but FC1/FCR2 sells for 10,000, and FC1/FCR3 sells in the millions of gold.
• Hit Chance Increase (HCI): Ranges from 1-15. HCI over 12, combined with DCI, is valuable.
• Luck: Ranges from 0-100, but since it can't be enhanced like armor, only keep it if it's 75 Luck or more.
• Skill Bonus: Ranges from 1-15. Only a few skills are in demand. Taming +12 or more since it allows control over beasts you wouldn't normally have is probably most in demand. Provocation and Magery are good, and Melee skills like Swords and Fencing are used for training. Some are not in demand at all, like Anatomy, Stealing, and Healing.
• Spell Damage Increase: Ranges from 1-12. This is a good add-on to a FC/FCR ring, but not very useful by itself.

Combos most people really, really want:

• Shield with HCI/DCI and FC1 for 1-handed paladins
• Shield with DCI and spell channeling without the FC-1 penalty for parry mages
• Jewelry with FC1/FCR3 and SDI
• LRC and High Resists
• Luck over 100 and High Resists
• Spell channeling weapons with high DI and HCI without Fast Cast -1
• One-Handed Luck spell channeling weapons without FC-1
• Jewelry with HCI/DCI/FC or HCI/DCI/DI
• Fast weapon with Lightning and SSI

Miscellaneous attributes few people care about

• Dexterity: 1-8
• Durability
• Enhance Potions
• Hit Dispel
• Hit Point Increase
• Hit Point Regeneration
• Intelligence: 1-8
• Jewelry with Resists
• Lower Requirements (Unless it's on something like a nice plate tunic)
• Mage Armor (only if the armor is good already, mage armor makes it a bit more valuable)
• Mage weapon
• Mana Increase
• Reflect Physical Damage
• Self-Repair
• Stamina Regeneration
• Strength
• Use Best Weapon (UBW)
• Nightsight