^ Im stalking you now
Using KP:
Buy KP during times of sale, you make the most profit when items are sold half the price at the kafra mall item.
W/O KP
Players mostly want 2 things in this game, costumes & gears. The latter includes anything that increases stats like runes, seedrunes, cards, upgrades, honed gears. Blue/Red seedrunes can be acquired from Fallen Fortress Mobs(lvl50 at least i think), groups of mobs that stun, silence, slow, poison, etc. Be warned. Also the Queen & Immortal Bosses drops them (you want Queen for more seeds) though these bosses spawn 4 hrs. Greenseeds from chaos though you need prereq quests to enter.
For cards, players say you combine 3 AoD card set then get 2 Osiris Cards for best results w/c you could actually wear even at lvl50. Don't bother farming below lvl50 cards just like below lvl50 gears. If you have played pre-AOV, STC cards or such cards and blue upgrade cards like Executer Meister & Abrahim are rendered obsolute. Even AoD cards tend to be obsolete since, Osiris Card got AP(more damage like green seed power rune), crit, vigor & penetration(ignores defense & good anti-tanks & players w/ +20 refined gears). And since AP is %, the higher your lvl & overall damage, the higher it will be. Hunt AoD cards though requires prereq dungeons & most likely you'll get green cards that you'll combine into blue cards that dont really sell nowadays. Move to farm Osiris Cards w/c is easily soloable even at ML11+.
Upgrades are the cost efficient in terms of getting your character in boost. Though don't bother below Osiris Gears. W/ ML30 Cap people will now look for Himmelmez's Gears upgraded or not. Use +10 Immediate Refine Stones since upgrading to get to a +10 is hardest than goin for 20. Then find a gold refine refiner for greater chance of success. Sell them gears or use them. PS: Luck required.
Honed Gears = Luck required. Also honing got more stats from 4 star gears but it's really luck. Some 2 star honed gears can beat them in stats. Gears are crafted and you need the recipe to start it w/. Get gold crafter from ranking for better crafted star results. During Osi i could solo and craft meself a 4 star. Can't solo Dark Whisper Bosses now though tanks got them pretty easily. You could try partying though like pre-AOV and get the loot via dice w/c is luck again or do it in a guild.
Costumes: Some want appearance, some for stats. Costumes have slots and can max to 3, and you can put up to +10 rune each. A hefty amount of stats if you get them all and a real boost. Depends on the buyer mostly for prices though handy if you've got something to sell that they really want.
I guess that's it. Also there are events that gives items most likely obtainable only from Kafra Shop. You could actually farm everything you want except some Kafra Shop only obtainable items. But people are too lazy to do that.