No, I'm not talking about the premiere stock market simulator for your NES console. I am talking about bartering goods for fun and profit (but mostly for fun), and breaking the game to do so.

Maybe this is a sickness, but I view it as something like solitare. It's a short break from the game you're playing, although using it in the games I mention can really be more of a chore than the game itself (due to subpar GUIs for trading).

A while back I was lucky to play the classic strategy/RPG game Fallout. Great game, but the bartering system got broken in a hurry, as with a bit of skill in trading you'd eventually be able to trade x number of item1 for y number of item2...and then you could trade y number of item2 + one or two of those funky green mutated apples (for example). It was a simple recursive process; if you were just a bit careful you could walk into the L.A. Boneyard's gun runner base with a fruit and a broken shotgun and walk out with all their guns, your broken shotgun, all their bottle caps, but no green fruit (SOMETHING has to stay in the trade, and the green fruit was usually it). You've got to be careful, though - trading away one Rad-Away too many can set you back about ten trades, if memory serves.

Pretty useless, but amusing.

I've also detailed a system for maxing your profits with the Haggle system in TES4: Oblivion here. It's not nearly as fun, but you don't have to click a little arrow fifty times in a row, either.

Also very useful in conjunction with this: The ITEM DUPLICATION CHEAT for Oblivion. Thanks go to A. Gardezabal/The Spaniard of GameSpy Forums/WhutDuFuk.com (yes really) for alerting me to this:

1.) Equip your bow and arrows. Draw back an arrow. Don't fire. Open the inventory.
2.) Go to the inventory. First *either* double-click (this is finicky) the arrows you've got equipped *or* equip a different type of arrow altogether (I found this works more easily, at least when I moved from say Dremora arrows to Dwemer, and then dropped the Dremora arrows). Now find the item you want to duplicate and shift-click (or drop) it. Again, this isn't terribly straightforward. Using the "all items at once" view might help.
3.) Arkay in heaven, the lag! I spent 10 minutes picking up something like 1000 Dwarven shortswords once, and probably just as long selling them, five at a time, to a merchant. It was like a giant briar patch of swords, and when I took one out the others on top would slide away, like pick-up-sticks or a Jenga game ("remarkably unlikable in many ways").

Having only a few of a type of arrow is a good idea here, especially if you're going to duplicate something like Sigil Stones (you've got to hear the head-splitting racket of 30+ Sigil Stones rolling around at once to truly appreciate how nasty that can be). Duplicating arrows is nice given that they stack into quivers automatically, cutting down significantly on the number of polygons that need to be onscreen. Note that many other types of items DO NOT stack in this manner, even if they appear to in the inventory menu. Potions, ingredients, and welkynd stones should stack, though, I think.

Note that use of the item duplication bug also allows the player to create barriers against enemies (though they'll lag you quite terribly as well), and frees you from needing to hop around like mad, picking all the flowers in sight. Need some more Aloe Vera? Launch it at a wall. It might not let you cheat in the Nirnroot quest (as I think the game increments a quest counter only when you find a Nirnroot, and wouldn't let you trade your ill-gotten goods to Sinderion), but it does let you duplicate nice quest items you want to keep ahold of.

Alright. That's enough from me. Feel free to share your system-exploiting adventures!