Those whom think 300 spaces is plenty must really be more of a solo type of player. I play every job in the game as do most since we are pretty much forced into it. When I'm grinding or out and about and get items for future crafting, why would I npc it or sell it to another person when I'm like going to have to pay double for it later. Granted I'm not at the point I need these yet, but will be there eventually.

Required Materials to keep as one of the LS Repairer's (Most common items only)
Weaver: cotton fent, canvas fent, velveteen fent, linen fent (4 items)
Goldsmith: brass nuggets, brass squares, silver nuggets, silver squares, electrum nuggets (5 items)
Blacksmith: bronze nuggets, iron nuggets (2 items)
Tanner: sheep leather spetch, buffalo leather spetch, toad leather spetch, boar leather spetch (4 items)
Carpenter: oak/yew/ash/pine/walnut/willow branches (6 items)
Armorer: bronze squares, bronze rings, iron squares, iron rings (4 items)

These are not including the uncommon repair items.

Rare/Important Item Examples: (stuff to keep for later grinding)
Mythril Ore/Nuggets and +3's
Steel Nuggets and +3's
Red Coral, Blue Coral, White Coral and +3's
Gold Ore, Gold Sand, Silver Ore, Silver Sand, Iron Ore, Iron Sand
Radiant Moraines, Moraines, Gemstones (Raw and regular and every HQ version of them)
Walnut Logs, Yew Logs, Oak Logs
Sheepskins, Boar Hides, Karakul skins, Peiste Skins

There are plenty more, just getting tired of typing these things out...

I myself keep things for future LS members who might need gear, usually HQ items of things I might already be done with.

I clean my retainers out about once a week and just npc extra crap that is no longer needed.

There is a need for more space, don't forget the 5+ sets of gear, And weapons/tools/ammo/potions you must keep. This kills off a lot of spaces.