Also I myself classify mobs into 3 "levels" or "tiers"
Level1: Sand, wood, etc. The easy ones, sandsmankings, scorpions, hobbits, hobbitmages, pirates, etc.
Level2: Slimes, sludges, ent ancients, werecats etc.
Level 3: Gods and mini-gods.
Monsteres go harder USUALLY the further you go into
Center of map. Usually "godlands" are in center, white snowy and grey areas in "middle". They are also the hardest from common mobs and give stat potions and useful items.
You trade at nexus, where it's safe. Items for items, or for potions. Here is price guide:
http://forums.wildshadow.com/node/368201 Speed potion: 10fg
1 Def Potion: 40fg
There you see the prices and you can convert them into fg etc easy by little calculating