So as I go around doing quests, I notice that the merchants change their inventory since I last looked, what triggers this?
There are three different kinds of bonus experience in Dead Island.
The first is bonus experience from breaking limbs. This is possible via blunt weapons and fists.
The second is bonus experience from cutting limbs. This is possible via sharp weapons.
The third, and last, is bonus experience from head-stomping fallen zombies.
Additionally, this bonus experience is scaled based on the level of the zombie, and is also affected by some skills in the characters skill trees.
There are a lot of prefixes and even more questions about them and their meaning. Some have only been seen in pre-release info or screeshots and some effects have not been well documented. There's a fairly good list on the Dead Island wiki and while it's a bit unreadable, the weapon mod FAQ lists the prefixes under the associated mods. Neither of these consider mods unique to one weapon as adding a prefix for the most part. If you can follow the association of the prefixes to the mods, the Dead Island wikia has some more detailed effect information. The accuracy of any of these sources is somewhat dubious for the effects so it may take some digging into the games' files for something more conclusive.
As you know, the game takes a base items and then adds prefixes after to modify characteristics. The game uses a weighted random algorithm with the item level as the base value to generate items and it is possible for items to have multiple prefixes. There are also prefixes from weapon mods that can be added and others of unknown origin.
I'll transcribe the known prefixes (107 at present) and their believed effects, ordering the generated prefixes according to the game's data file and I'll add whether I believe the descriptor is positive or negative:
Name Effect Defective -Accuracy Skewed -Accuracy Imprecise -Accuracy Flawed -Accuracy Unreliable -Accuracy Short range -Accuracy Semi-accurate +Accuracy Well-balanced +Accuracy Calibrated +Accuracy Accurate +Accuracy Precise +Accuracy Trusty +Accuracy Spot-on +Accuracy Perfect +Accuracy Dead Eye +Accuracy Feeble -Damage Puny -Damage Mediocre -Damage Ordinary -Damage Spiteful +Damage Malicious +Damage Damaging +Damage Dangerous +Damage Savage +Damage Vicious +Damage Cruel +Damage Slaughtering +Damage Devastating +Damage Deadly +Damage Lethal +Damage Flimsy -Durability Frail -Durability Shoddy -Durability Fragile -Durability Weak -Durability Worn -Durability Used -Durability Reliable +Durability Dependable +Durability Sturdy +Durability Stout +Durability Strong +Durability Tough +Durability Impervious +Durability Indestructible +Durability Tiring -Force (may lower handling?) Frightening +Force Striking +Force Threatening +Force Debilitating +Force Awe-inspiring +Force Formidable +Force Terrifying +Force Disabling +Force Shattering +Force Enervating +Force Crippling +Force Incapacitating +Force Paralyzing +Force Diabolical +Force Sluggish -Reload Slow -Reload Unwieldy -Reload Languid -Reload Medium-speed -Reload Brisk +Reload Rapid +Reload Quick +Reload Swift +Reload Deft +Reload Speedy +Reload Fast +Reload High-speed +Reload Super-quick +Reload Lightning-fast +Reload Critical Crit +? Weighted +Damage, +Force, Crit +4% Shark +Damage, +Force, Crit +6% Heavy +Force or +Handling or +Force, +Damage, -Handling, Crit +4% Piranha +Damage, +Force, -Handling, Crit +4%, Bleed out chance Barbed Bleed 1, Crit +4% Bleeding Bleed 1, Crit +5% Naild Bleed 1, +Force, Crit +4% Bolted Bleed 1, +Force, Crit +4% Glazed Bleed 2, Crit +4%, Bleed out chance Razor Bleed 3, Crit +4% Slash Bleed 3, Crit +4% Sawdisc Bleed 4, Crit +6% Explosive Explosion 2 Burning Fire? Striker Fire 1, Crit +2% Scorching Fire 1, Crit +4/5% Phoenix Fire 2, Crit +6% Pride Impact 1, Crit +1/2% Impact Impact 1, Crit +4% Lightning Impact 2, Crit +4% Magic Wand Impact 3, Crit +4% Tesla Impact 4, Crit +6% Detox Poison 1, Crit +1/2% Paralyzing Poison 1, Crit +4%, Disabling chance Venom Poison 2, Crit +4% Toxic Poison 3, Crit +4%, Corrosive chance Deathstalker Poison 4, Crit +6% Shock Shock 1, Crit +1/2/4% High Voltage Shock 2, Crit +4% Short-Circuit Shock 3, Crit +4% Old Smoky Shock 4, Crit +6%
There are still a lot of unknowns pertaining to the prefixes. I've seen the burning prefix shown in screenshots, but I'm not sure if they've actually been found and exist within the game or if they were part of the pre-release screens. It is listed on the wiki so I added it, but information on these is scarce. There's also the question of effects on handling as few of these seem to provide their clear effects on it as the critical mods seem to reduce handling and the tiring prefix has been described as doing this as well, but the effect is hard to corroborate.