For those who are still having problems on inventory spaces, being unable to add any items in their inventory because it is already full or nearly full, here is a guide to free up a lot of space. There are items which could be sold, items which could be broken or both. Sell those unuseful items or break them as you wish to free up your inventory space. Item breaking, of course is thru the use of DK-P Type 0812 or more commonly known as the crafting/breaking machine on your grand chase Menu tab while you are logged in the game.
As we know (and for those who don't please read) breaking items will give off crafting materials namely Inshentyum, Noksyum and Asgadyum. This materials are used to craft armors and weapons similar to those which are dropped in dungeons and for some items that are not droppable, with stat/grade as normal, rare & epic even legendary for the transcend and eclipse.
Now, questions will follow on which items to sell/break and so this guide is created to make the selection process easier and guilt free.
So now I shall be sorting items into general categories and tell you how to deal with each. This guide is solely based on opinions, so if you think you know better, by all means do what you want. I'm not being sarcastic, since item usefulness varies per individual.
P.S. Keep a mentality of what items you REALLY (can't be stressed enough) need. It's basically the essence of this guide. Knowing what items you currently have, plan for a final build, and will use to achieve that final build will help you.
Items you SHOULD NEVER EVER sell
Emoticons - They eat a lot of visible space but never really consume inventory space. If you still sell them since you hate seeing a wall of emoticons, this guide might not be for you, but I advise you to use the Etc. function. It lets you see most of the items beyond the emoticons without scrolling down.
Beads and Spell Scrolls - These gacha related items you may unintentionally receive don't eat space, but their prizes do.
Bags - You probably have max space but you should still keep these.
Missions - Register them instead.
Items you SHOULD NOT sell
Accumulated items - Items like gems, aging cores and fragments are easy to obtain. Except you may need hundreds if not thousands. In that case don't sell them. If you hate crafting then ditch fragments altogether.
Coin of Heroes (Piece) - There's a maximum to how many of these you can obtain per day, so don't waste that time just for space.
Bonuses - Use them instead (unless you already have 99 charges, in that case you may opt to delete them because they're easy to replenish).
Cash items - Selling/breaking items you bought is kinda wasteful, even if you used event money. But come the time when you'll be able to buy higher level or stat cash items you have the option to break those old cash items since if they are equips then it will give crafting materials that are pure and these ones are hard to collect. Anything purchasable from the item mall counts, even if they were freebies (nickname change cards).
Champ tickets - If you play a lot enough to worry about your inventory, you'll probably get 3 tickets every day. It'd take more effort to sell them, and keeping them keeps you ready for champ runs.
Current equipment - Unless you like running around naked. Consumables included. Items you plan to equip also count (except NPC equips).
Useful timed items - Most obviously GC Club and Nutrients IV.
Items you SHOULD break
Costumes - One or two sets are enough now that the avatar system have slightly started. Looking cool in battle is justifiable, showing off for others is doing it wrong. Especially when you can dress up your character in the shop. Limited items and taking shots of your character doing a move while wearing a costume are exceptions, but seriously, who are you trying to impress?
Freebies - Face it, they're useless. Signboards make up most of these, but even if they're not signboards and seem useful (zero mini pet for example), delete them. The only good stuff usually come in sets (like Soul Mate accessory set) although you'll outgrow such sets by the time you'll be having cash items which have higher stats than those free ones.
Lower-tier items - Sell brave runes when you use honor runes, small potions when you use large potions. Armors too, who cares if you have a ton of epic limited or farmed equipment if a single epic gacha set outperforms them all in style and functionality? Don't get started with that lame B+ excuse again (there are always GP sets), and stuff like SD Toilet Head are jokes for the park (a single avatar set is alright though, but a ton of avatar sets are just for jerks).
You don't need several epic gacha sets for one character either - 4 at maximum if you actually collected all epic weapons (which is fine), 5 if you want to try all possible property combinations (which is dumb, especially if you have pieces with repeating properties, unless due to some stats fix and the addition of Hell Spear Damage ). No wonder cash freaks sob over their inventory.
Non-stacking items - These are culprits that you may not notice right away. Sometimes you'll have two sets of the same item, for example one set contains 1 normal SP reset card and the other set contains 12 epic SP reset cards. They're both reset cards but consume 1 space each, you may want to delete or use the other lone card.
Pets - You collected every pet in the history of GCPH yet you only keep using one of them. If you really went that far then I won't tell you to stop, but if not then get rid of your show pets. Pet books that are more than one for each kind of pet are also candidates for deletion. Use one pet of each kind not multiples.
Pet attacks - Sell these when not in use. 2k gp per 100 charges is cheap to replenish.
Training equipment - Stuff you get from job changing. Beyond aesthetics, if you worry about your inventory then you probably have enough money to buy and sell NPC equipment for temporary use.
Adventurer's set - This deserves a special mention. You get 7 pieces of trash (costume if you like, but still) per character specially when a character is newly aquired. Conversely, you can gain 7 space per character by deleting all of these, and that's a lot.
Useless rares - You have an evilis weapon and a thunder hammer weapon. If you're too dumb to figure it out then I'll kindly instruct you to break the evilis weapon.
Useless timed items - You got the monthly attendance accessories, except you have some limited edition accessories that also get stronger based on your level, are permanent and lose only by a few stats. Don't wait for two weeks, just delete the monthly accessories.
Items with charges - Items that are seemingly useful but actually useless like the gear second ring and for those who still have these season 3 consumables like the kazeaaze's healing ball, lena's artifact, true berserker's ring and oxygen and gas masks. When justified, you may keep some of these. However, you may experience this paradox - you save them for future use, yet you feel guilt when you want to use them, and you'll never really know that it's time to use them until that time has passed. Unquestionably, sell the useless masks, and think twice about keeping the useful other stuff. You won't be equipped with them forever, so might as well start practicing how to fight without them.
So there it goes, anyone could reply on things they could add or things they want to correct from my post. Just remember not to hug your items too tightly.
Credits to SoennoKaito