(In case you've been wondering where I've been, the 3DTotalgames website exploded a little while back and I've been trying to cobble it together into something that works again. It's there, but it'll take me some time to sort out all of the details, however we should be back to weekly updates for the forseeable future)
When I think about the things that add the most to my life I think about hugs, time spent with friends, time spent creating things and stuff of that nature. Most of it has a common theme that I think is important in a well lived life: They're often interactions from which everyone gains. If I take a pound from you then you are one pound poorer, but if we have a hug then we both gain one. Which lead me to think about the nature of trading, something that should be an activity by which everyone gains. Different things have different value to different people and so it seems obvious that it's often possible to make exchanges such that everyone involved leaves with more than they started with - in a sense that's meaningful to them. Usually.
Trading well involves a lot of skills, being able to identify what you have that's of limited value to you and what your partner has that's valuable to you but worthless to them are tricky things. There's also a need to adopt some moral framework, or at least to convincingly fake one, if you intend to trade with the same group more than once. Which in turn makes noticing that your partner hasn't traded with the same group more than once worthwhile too. On top of that sit all other aspects of negotiation as a more general skill. With so much inherently involved it's proven a fruitful area for game designers, as a fairly minimal set of rules can evoke quite complex thought and behaviour.
Like it or loath it almost everyone who's into games has at some point played Settlers of Catan. It gets a lot out of its trading mechanic, which emphasises all other elements of the game. The choice between getting the resource you need for two goods and making another player more powerful and getting it for four, but without aiding a rival is an interesting one. The need to keep a small hand size in case the robber moves acts contrary to accepting otherwise desirable trades. Trading helps to mitigate against a starting position that's lacking in some critical element. There are also times it can tie into the mechanics in the other direction, such as the reluctance to trade wood or bricks to a player who is racing against you to build a particular road, which can wind up in "I'll match that offer for you to not give it to him."
Trading mechanics can differ substantially from game to game, the sort of trading that occurs in the real time game Pit is very different to the more considered options that occur in Settlers. There's also a need for game designers to consider things outside of the strict rules. I had a memorable game of Twilight Imperium in which one player started offering to loan people trade goods, for a greater return on the following turn.
Nothing in the rules supports this, but nor does it prohibit it. Any of his trading partners could have just kept his goods on a subsequent turn and the game would have done nothing, but the need to have other players trust your word kept most people honest most of the time. This provided a fairly significant advantage, as he would make his offers just as players were realising they'd need (for instance) just a few more resources to take advantage of the secondary research power or some similar opportunity. It also provided a strong board advantage, as he made more generous offers to the neighbours of his neighbours, giving them more to worry about in terms of the quantity of forces on their immediate borders. It was an interesting play that shaped the game
Which of course doesn't make you a good person if you do it outside of the gaming table. My point is that game designers need to account for the unintended consequences of their rules as much as for the intended ones, "the players are playing it wrong" is rarely if ever a valid response to playtester criticism. This is particularly important for trading mechanics, as any time players are allowed to trade resources, 'future behaviour' becomes one of the resources to be traded unless expressly prohibited (and sometimes even then!)