What makes workers a commodity that may be traded in diplomacy? Why can't we trade other units? Is this feature totally hardwired into the game? Or can it be modded around?

Also, why won't the AI ever make any deal involving cities? They won't even take one from me. I have often wanted to stop sacking an entire civilization (for sentimental reasons) and just make them give me their overseas colonies, but they steadfastly refuse to accept them. Is there any way to change this?

Also, what about the AI that refuses to give me more than about 300 gold for any resource, but often demands 1500 gold for their own resources. Is there any way to adjust this? How is the price of a resource actually determined? It doesn't seem to be a supply/demand thing. No matter how many civs have tobacco, they all seem to want 1500 gold for it (or more). The most absurd request I have seen yet was an offer of silks in exchange for wine, furs and only 200 gold (per turn!). Well, I told the Byzantines to shove it, then I sacked them. Is there any way to adjust this?

PS - if these questions have already been asked, my apologies, but it is hard to search this forum.

I don't know about the worker thing , but about trading cities I do. The AI only trades small unimportant cities. And only then when you are bashing them at war. Usually when I have an enemy on their knees, I will offer a peace treaty in exchange for their smaller cities. I actually got 15 cities in a Tundra area and only left 3 cities to that civ. They were big cities and one was the capitol so I had to come back later and take those by force.

The resources I can't answer either, except that they will only rtrade you good money for them when they really need them. That isn't often.

Your workers have to be in your capitol city in order to exchange them in the diplomacy screen

And, on the editor's "General Settings" tab, just below the "starting units" section, there's something like a "tradable unit" or "slaved unit" line... If you change it to, say, the archer, you'll be able to trade your archers with other civs, if they're on your capital. But, every 0-defense unit you beat, such as workers or catapults, will become archers as you defeat them.

LBPB: I believe he was wondering how you get to be able to trade other units, and he knew how to trade workers. How do you what graphics to use for the enslaved worker?

No, you only get the 'captured unit' if you defeat a unit that has 0 defense like the worker/catapult/settler... AND population cost bigger than 0. So if you capture a worker, you get a captured unit (worker in default rules) because it has pop cost of 1, if you capture a settler you get 2 workers cause the settler has cost of 2, and if you capture a catapult you get the same catapult cause it has a pop cost of 0.