The treasure system is responsible for the various items left behind by creatures after LOOTing them. The treasure system will enhance treasure drops for creatures which have not been hunted much recently, and will do the reverse if they're been hunted continuously. In addition, if a creatures is higher or lower in level compared to the group of characters hunting them, the value of their treasure drops will be enhanced or diminished, respectively. This governs the apparent wealth of creatures, rather than any particular creature being intrinsically "rich" or "poor."
F2P subscriptions will find considerably less treasure than paying accounts.
What you can find
There is a large diversity of items to be found. Although there are a variety of more 'useless' items like a rusty doorknob, chances are if you don't know what something less mundane is for, it might be worth something. Some items are definitely worth selling to shops in your local town, while others may be more coveted by other players and worth holding on to for a better return. Below is a basic list of the types of items which are commonly found:
Where to sell your loot
The local pawnshop will buy just about anything, but tends to offer a lower price than a competing store which may also purchase a given item. Some choice finds to pawn are clothing and containers, especially the nicer looking ones like silk shirts and brocade cloaks; keep in mind items like bodices which are pin-worn with a couple small pockets will surely fetch a higher price from a fellow player. The two other places to hock your goods are the gemshop and the consignment room at the local alchemist.
Nearly anything with gems or precious metals in it which isn't combat gear (weapons, armor, etc) or lockpicks will fetch about three times more coins at the gemshop than the pawnshop; this ranges from glittering brooches to fancy mithril dishware and mantelpieces like statues and figurines. In fact, the offset between the prices is so large, one can actually buy these items from the pawnshop—which others have foolishly sold there—and turn a tidy profit on a slow morning selling them directly to the gemshop instead. On the topic of precious metals, silver lockpicks get a reasonable price at the pawnshop and are usually worth picking up.
Most the other strange items you see may be alchemical reagents if they aren't standard magic items like wands and colored crystals. The varieties of ayanad crystals can be found on most any kinds of monsters (depending on the crystal and the creature level range), yet other things will be found more specifically; pristine nymph's hair would be found only on sea nymphs, not mountain trolls, for instance. The more common items range from colored essences and dusts to a variety of cores, and much more rare items. These should rarely be left on the ground, and at the least sold to the consignment at the alchemist. If it's an item only found on one or two creatures in the game, and you are planning to hunt that area for awhile, getting a jar from the alchemist to hoard them (jars only take up one slot in your locker) can give quite a good return for the investment of a little time selling the full jar to a fellow player later on.