I NPC/drop a whole load of crap every time I go out on the field yet my main character is always about 250/300 and my mule 100/300. Why? Because I'm a goldsmith, a carpenter, a miner and a botanist. What this means is I need to hold on to all the raw gems I get NQ/+1/2/3, gems I've converted, every quality and every type of iron/silver/gold/mythril, HQ3 zinc and copper, every worthwhile log, HQ branches, various lumber of various qualities. Plus other mats that are used for crafting finished products for these crafts such as various glues, squares, nails, rivets, horns/bones etc. Then I've got the backlog of products to put up for sale that only a retard would NPC like buffalo hides, toad skins etc. Then I've got broken gear that needs vintaging at some point and the 70 or so slots my character's equipment takes up for my classes.
Now I could easily sell the iron/silver/gold/logs I'm not currently using but they're worth more as mats than gil since prices are only going up. Iron alone probably takes up about 40 slots on my mule but I'm saving that as BSM will be my next major craft.