What is a good strategy for finding and rerolling charms?
In all my time playing Diablo 2, I've found few very good charms (excluding the uniques). The market seems to be flooded with incredibly good charms - +1 to class skill tree, resistances, life, magic find, you name it. I've heard mention that it is possible to re-roll charms, and I've looked up the formulas. However, I know that level has a lot to do with the possibilities involved in the randomly re-rolled charm.
What is a good strategy for finding and re-rolling charms with the express purpose of creating high-power ("godly") charms for selling?
Simple steps
Step 1: If you know what sort of charms you are looking for, you can use the charm affix list. This will give you the item level requirements for your specific charm. If you are looking for both a prefix AND a suffix, take the highest item level. Your character's level must be at minimum 75% of this.
Step 2: Find an area that has monsters that are at least the item's level. For the higher level charms, bosses may be the only option. Area level reference here. As shown, max area level is 85, And even Hell Baal is only level 99. So any item level of 85 or higher can only be found in the Worldstone Keep (Hell) and onwards. eg. One of the suffix`s is Vita, with an item level of 77 or 91. Since the only monster above level 91 is Hell Baal, getting this suffix means any charms he drops are level 91, and have a chance of the higher Vita suffix. Any other monster that drops a Vita charm will have less life (36-40 as opposed to 41-45).
Step 3: Brutally murder any and all monsters that meet these criteria. There may be some collateral genocide. This is usually to your benefit. Hope a charm drops from these monsters.
Step 4: Continue committing genocide until enough charms have dropped. Bonus if you collect lots of gems, too. You'll need them.
Step 5: After identifying charms, see if any provide what you're looking for.
Step 6: Insert charm and three perfect gems into your cube and hit Transmute.
Step 7: Does the charm provide what you wanted? If so, success! If not, repeat step 6.
Item Level
Item level is a hidden stat for every item. It dictates what affixes are possible for the item to have. Whatever items a monster drops are the same level as the monster. eg. Monsters in Worldstone Keep (Hell) are level 85. Therefore, all the magic items they drop are item level 85. Note: This does NOT guarantee that the items they drop will be better than what you currently have. They have a CHANCE of rolling a higher level affix.
Example: The prefix Serrated is item level 67. This means that any monster at level 67 or higher has a chance of dropping a charm with the Serrated prefix. However, the monster might drop a charm with the Forked prefix, which requires item level 37. This does NOT mean the item's level is 37. The charm's item level is whatever level the monster was. This is important to remember. When re-rolling the charm, it still has a chance of generating the Serrated prefix. Not a high chance, but a chance. Whereas a monster of level 37 can drop the same Forked charm, but has NO chance of generating the Serrated prefix, ever.
Extra explanation
Recipe: 3 Perfect Gems + 1 Magic Item → 1 Random Magic Item of the same type
The item level will match the inputted item, so you already need a charm that's of a high enough level to get the affixes you want. You can look here to find out what the minimum item level your current charms are. (warning, the list is huge) Look above for the explanation on item level.
Once you have a charm you don't want that's a high enough level, it's all a matter of luck.
Simplest example: If you're looking for a +1 to a skill tree, these are all item level 50. There are no other charms that have an item level of 50, so for the best chance of getting the tree you want, you'll have to find a grand charm that gives only +1 to a tree you don't want, and start rolling. The chances of this happening are roughly 10%. For the math (lots of math), go here. These are your best chance for finding + to skills.
Grand Charm Affix List
Note: I`m going to be limiting this list to grand charms only. The other lists can be found at the above affix list link.
Prefixes Attributes lvl
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Stout +3-5 to Defense 1
Stout +6-9 to Defense 7
Stout +10-12 to Defense 12
Burly +13-15 to Defense 17
Burly +16-22 to Defense 22
Burly +23-30 to Defense 27
Stalwart +33-40 to Defense 32
Stalwart +44-50 to Defense 37
Stalwart +60-100 to Defense 42
Red +1 to Minimum Damage 15
Sanguinary +2 to Minimum Damage 45
Bloody +3 to Minimum Damage 75
Jagged +1 to Maximum Damage 7
Forked +2 to Maximum Damage 37
Serrated +3 to Maximum Damage 67
Rugged +12-24 Maximum Stamina 1
Rugged +25-36 Maximum Stamina 7
Rugged +37-50 Maximum Stamina 21
Bronze +6-12 to Attack Rating 1
Bronze +13-27 to Attack Rating 7
Bronze +28-42 to Attack Rating 13
Iron +43-57 to Attack Rating 19
Iron +58-72 to Attack Rating 25
Iron +73-87 to Attack Rating 31
Steel +88-102 to Attack Rating 37
Steel +103-117 to Attack Rating 43
Steel +118-132 to Attack Rating 49
Fine +10-20 to Attack Rating
+1-3 Maximum Damage 15
Fine +21-48 to Attack Rating
+4-6 Maximum Damage 22
Sharp +49-76 to Attack Rating
+7-10 Maximum Damage 29
Lucky +1-4% Better Chance of Getting Magic Items 21
Lucky +5-7% Better Chance of Getting Magic Items 38
Lucky +8-12% Better Chance of Getting Magic Items 55
Lizard's +3-7 to Mana 1
Lizard's +8-13 to Mana 7
Lizard's +14-20 to Mana 13
Snake's +21-26 to Mana 19
Snake's +27-33 to Mana 25
Snake's +34-39 to Mana 31
Serpent's +40-46 to Mana 37
Serpent's +47-52 to Mana 43
Serpent's +53-59 to Mana 49
Shimmering All Resistances +3-6% 8-13
Shimmering All Resistances +8-12% 14-34
Shimmering All Resistances +13-15% 35
Azure Cold Resist +7-15% 1
Lapis Cold Resist +16-20% 10
Cobalt Cold Resist +21-25% 20
Sapphire Cold Resist +26-30% 30
Crimson Fire Resist +7-15% 1
Russet Fire Resist +16-20% 10
Garnet Fire Resist +21-25% 20
Ruby Fire Resist +26-30% 30
Tangerine Lightning Resist +7-15% 1
Ocher Lightning Resist +16-20% 10
Coral Lightning Resist +21-25% 20
Amber Lightning Resist +26-30% 30
Beryl Poison Resist +7-15% 1
Viridian Poison Resist +16-20% 10
Jade Poison Resist +21-25% 20
Emerald Poison Resist +26-30% 30
Fletcher's +1 to Bow & Crossbow Skills (Amazon Only) 50
Acrobat's +1 to Passive Skills (Amazon Only) 50
Harpoonist's +1 to Javelin & Spear Skills (Amazon Only) 50
Burning +1 to Fire Skills (Sorceress Only) 50
Sparking +1 to Lightning Skills (Sorceress Only) 50
Chilling +1 to Cold Skills (Sorceress Only) 50
Hexing +1 to Curses (Necromancer Only) 50
Fungal +1 to Poison & Bone Skills (Necro Only) 50
Graverobber's +1 to Summoning Skills (Necromancer Only) 50
Lion Branded +1 to Combat Skills (Paladin Only) 50
Captain's +1 to Offensive Auras (Paladin Only) 50
Preserver's +1 to Defensive Auras (Paladin Only) 50
Expert's +1 to Combat Skills (Barbarian Only) 50
Fanatic +1 to Combat Masteries (Barbarian Only) 50
Sounding +1 to Warcries (Barbarian Only) 50
Trainer's +1 to Summoning Skills (Druid Only) 50
Spiritual +1 to Shapeshifting Skills (Druid Only) 50
Natural +1 to Elemental Skills (Druid Only) 50
Entrapping +1 to Traps (Assassin Only) 50
Mentalist's +1 to Shadow Disciplines (Assassin Only) 50
Shogukusha's +1 to Martial Arts (Assassin Only) 50
Septic +5 Poison Damage over 3 seconds 1
Foul +15 Poison Damage over 4 seconds 12
Toxic +50 Poison Damage over 5 seconds 27
Pestilent +100 Poison Damage over 6 seconds 42
Snowy Adds 1-(2-3) Cold Damage (1 second Duration) 9
Shivering Adds (2-4)-(4-8) Cold Damage (1 second Dur) 15
Boreal Adds (5-7)-(9-15) Cold Damage (1 second Dur) 30
Hibernal Adds (8-13)-(16-25) Cold Damage (1 sec Dur) 45
Ember Adds 1-(2-3) Fire Damage 5
Smoldering Adds (2-3)-(4-7) Fire Damage 15
Smoking Adds (4-7)-(8-19) Fire Damage 28
Flaming Adds (8-18)-(20-36) Fire Damage 43
Static Adds 1-(4-5) Lightning Damage 7
Glowing Adds 1-(6-17) Lightning Damage 14
Arcing Adds 1-(18-44) Lightning Damage 29
Shocking Adds 1-(45-79) Lightning Damage 44
Suffixes Attributes lvl
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Balance 12% Faster Hit Recovery 1
Frost Adds 1 to 2 Cold Damage - 1 sec Duration 5
Icicle Adds 2 to (3-4) Cold Damage - 1 sec Duration 8
Glacier Adds 3 to (5-7) Cold Damage - 1 sec Duration 15
Winter Adds (4-6) to (8-11) Cold Damage - 1 sec Dur 23
Flame Adds 1 to 2 Fire Damage 3
Fire Adds (1-2) to 3 Fire Damage 8
Burning Adds (2-3) to (4-7) Fire Damage 14
Incineration Adds (4-7) to (8-14) Fire Damage 22
Shock Adds 1 to (2-3) Lightning Damage 4
Lightning Adds 1 to (4-7) Lightning Damage 7
Thunder Adds 1 to (8-21) Lightning Damage 15
Storms Adds 1 to (22-33) Lightning Damage 22
Blight Adds 6 Poison Damage Over 3 Seconds 1
Venom Adds 15 Poison Damage Over 4 Seconds 6
Pestilence Adds 25 Poison Damage Over 5 Seconds 14
Anthrax Adds 50 Poison Damage Over 6 Seconds 21
Craftsmanship +1 to Maximum Damage 1
Quality +2 to Maximum Damage 41
Maiming +3-4 to Maximum Damage 71
Inertia 7% Faster Run/Walk 19
Greed 10-20% Extra Gold From Monsters 1
Greed 21-30% Extra Gold From Monsters 7
Greed 31-40% Extra Gold From Monsters 19
Dexterity +3-4 to Dexterity 1
Dexterity +5-6 to Dexterity 14
Life +5-10 to Life 1
Life +11-15 to Life 9
Life +16-20 to Life 19
Sustenance +21-25 to Life 31
Sustenance +26-30 to Life 45
Sustenance +31-35 to Life 61
Vita +36-40 to Life 77
Vita +41-45 to Life 91
Strength +3-4 to Strength 1
Strength +5-6 to Strength 14
@fbueckert has covered everything already, I'd just like to suggest that it also depends on the char - get a character good at mowing down mobs, suitable Act 5 Hell place to kill them, and just count on the large number of drops to include a good haul of charms (that you can then reroll if needed). It won't be the highest level like Baal drops but good enough for +skill mods.